::Asami (!hugAsamiok)'s quick & dirty anatomical guidelines for creating your own shape::
Before we begin:
-Do not rely on the shape editor's description of how tall you are. It is wrong. Find a good in-world tool, or measure yourself against a prim. I have a height ruler I will gladly give out if you need one.
-This guide is tailored for people that want to look idealized, anatomically accurate, and realistically scaled. Phat-azz users need not apply. You will always look terrible. It is also tailored to Caucasians since those proportions can be used for a majority of non-Black/African and non-Asian races. Look at pictures, it is not hard to point out the differences and adjust as necessary if you are making a non-White character.
-ALWAYS USE REFERENCE. Google pictures of people you find attractive or want to look like, ESPECIALLY if you're going for a Black/African or Asian look. There are both subtle and not-so-subtle differences to how their faces (and sometimes bodies) are structured. You will look silly if you don't.
-Finding a skin that looks good is almost as important (perhaps even more important) than the shape you use with it. It is highly recommended that you have an idea of what skin you want to use when creating a shape, as makeup/etc. can change how it looks and will require tweaks to the face. However you can get a pretty good baseline shape which you can use for anything and tweak later by following this guide.
-A cursory Google Search on anatomy for art will teach you far more than this guide ever can, this is the tl:dr version.
Step 1 - Body:
This is the easier part, and it's a lot harder to screw up, so we'll tackle it first before getting to the details. When creating a shape you should be naked, devoid of attachments, and devoid of hair. Don't rely on a hair to make your head look right, you will almost always end up with THS (Triangle-Head Syndrome) that way. It is a crippling, debilitating disease that affects many mainlanders.
Manly men use a male base shape, girls and girly boys use a female base shape. If you want to look fairly ectomorphic or lean as a boy, without being full on girly boy, pay attention to the girly boy guide but use the male proportion guidelines and adjust as necessary.
It's also important to note that tweaking is a must with this stuff, everything relies on everything else for proportion, so changing one thing to make it look better often necessitates changing other things as well. However, much like in the real world we are not all perfect, so it's best to learn to let the little things slide. None of these guidelines are set in stone, the key is learning how to exaggerate tastefully and skillfully.
Think of a good middle-of-the-road height you want to be, and get your default shape around there. Remember, use an in-world tool to measure, because the one listed in the shape parameters is wrong. Using the height slider for this is ok, but it's better to use it for very small touch-ups later. You probably don't want to bring it all the way down, also, unless you're trying to create super-short (<5') or child avatars.
Once you have a good baseline, it's time to adjust the larger proportions.
Your hips should sit roughly midway between the top of your head and the base of your feet, they are the mid-point of your body. If you're a girl or girly-boy, you can skew this slightly so that your legs are a bit longer (to make yourself look more 'leggy'), but not too much. Skewing it the other way is not advisable, as it tends to look derpy very quickly. Use Torso and Leg length for this.
Your arms should go down long enough that your fingertips reach just about mid-thigh. There's no real benefit to making them shorter or longer, but you could if you really wanted.
For girls/girly boys:
Hips come out to about the same width of your shoulders. Finding your shoulder width is a little trickier, since it relies on your head size. This is one of those annoying parts where you may want to go back and forth finding a good balance between your body/shoulder width and head size. The guideline is roughly two head widths. If you're looking for that super round hour-glassy style shape you see so much: Don't use your hip-width slider to go all the way. Take your hip-width in and use the Saddle Bags slider to reach the full width. It's less-realistic but more desirable for most people.
For manly-men who like pick-up trucks and guns:
Hips are roughly 2/3rd the width of your shoulders. Like for girls, finding your shoulder width is a tricky balance with your head size. You're looking about 2 head widths for a lean/ectomorph build, and 2.5 or so for a Mr. Universe build. Take it all the way to 3 head widths if you're -really- trying to compensate for something.
This measurement is about the same for men and women. Your waist comes in to about nipple-width. If you're extra manly you might extend it a bit more, but it tucks in a little on males as well, usually. I wouldn't recommend pulling it in further, but if you want to look thicker/more girl-next-door-y, you could leave it out a little.
This is one most people get wrong in Second Life. People want small, dainty hands, and it usually makes the wrist of the LL body look even worse than it does by default. I usually eyeball this and do ok. However if you want a proportional measurement: My hand with fingers spread is roughly 2/3rd the length of my forearm from wrist to the tip of my middle finger. Check your own to see if it's similar. Start there at the very least.
Feet for females and girly boys should always be at 0. For males you basically want to use whatever fits in your shoes. This is probably also 0, or close to it (5-10 perhaps).
This is largely a matter of preference. For girls and girly boys I recommend a value between 30-45. For manly men the sky's the limit. Keep in mind that the more muscular you are the more jagged and therefore crappy you look. If you really want to be super-body-builder mode, you might be better off with a mesh body.
Boobs & Pecs:
For girls, I wouldn't recommend using a breast size over 60, the mesh starts to get very badly deformed past this point. If you want big boobs, you should probably just use mesh/sculpt boobs. If you're going for regular size however, between 35-50 is a good size. The most important thing to remember is to keep them realistically positioned. Breasts have weight and do not sit perfectly sticking out of your chest. Look from the side and try to make a smooth curve from your collar-bone around the breast using the buoyancy slider. Also remember that nobody has natural cleavage. There should be space between your breasts when working with the breast cleavage slider. That's one you really have to eyeball. Use the nipple-waist proportion as a very rough guideline.
For girly boys, you pretty much want to take the breast size slider all the way down. I wouldn't recommend a dead 0 though, something around 5 is usually best, this prevents your chest from looking sunken in. For buoyancy and cleavage sliders, just fiddle with them until you get the nipples in the right spot, and minimize stretching of the texture.
For pecs on manly-men, I'd recommend keeping your pectoral slider between 45 and 65. Any less and you start to get really weird deformations, any more and it looks super sunken-in. Aside from that it's a matter of preference. Again, the more muscular, the more jagged, and thus the more crap it looks.
Step 2 - The Head:
This is where shit gets real. Use tons of pictures, have a good idea what you want. It's the difference between falling into Down's-syndrome face or skirting the edge of the uncanny valley at the pinnacle of Linden-beauty. I should also note that because there are so many controls here, I won't be going over specific sliders as much. It's up to you to play around and find the best combination of them to achieve the results you want. Crafting a Second Life face is far more about the positions and sizes of things than slider values.
First things first, get your head shape right. Look at pictures of skulls, use the upside-down egg rule, etc. Look at pictures of bald people. Remember that your head is not going to be triangular, even if you have a rounder face. There aren't really any easy-to-follow reference guidelines for this stuff, so you're sort of on your own. It really shouldn't be that hard however. How many dozens of times a day do you see a head?
Eyes sit roughly midway down a human head, and are sized and positioned in such a way that, looking from the front, you could line up roughly 5 eyes across the width of the face (the two on either side of the center being the ones we see, obviously). The short of this is that you should have room for an additional eye between your eyes, and an additional eye between your eye and the side of your head. You can probably get away with making your eyes slightly bigger, if you like. However for girls remember that the makeup on your skin can often do this for you.
Another thing to remember is that the upper-lid of your eye should be more curved than the lower lid. I find an easy way to do this is to pull the inner eye corner almost all the way down (maybe 5 ticks up from the lowest), and then pull the outer eye corner a little less downward than that. Pulling it down farther than the inner eye corner will make your eyes look tired or droopy, so don't go too much.
For eye sunkenness, refer to pictures, that's really the best way to see how sunken to go. You could probably leave this alone in most cases anyway. Same for bags and puffy eyelids. If you're going to adjust it, basically just use whatever has the least texture stretching, unless you're going for a specific look.
The bottom of the human nose sits roughly halfway between the center of the eyes and the bottom of the chin. The width of the nostrils should line up with the inner corners of the eyes (making it roughly one eye-width). The bridge of the nose is largely a matter of preference, as is how far the nose sticks out. Smaller, more up-turned, small noses look more dainty and cute, larger, more downward pointed noses look more manly.
A special note, however: There is a rough shading patch at the sides of the nose where it meets the jowl lines. This can be partly alleviated by turning the nose more upward. You should find a way to balance between poor face shading and a too-upturned nose for best results. It's up to what you want out of your avatar, however.
The human mouth sits roughly halfway between the bottom of the nose and the bottom of the chin, and the width of the mouth should extend to the center of the eyes (making it roughly two eye-widths). You can get away with making it a little shorter, but making it longer tends to make you look weird very quickly. For a pouty look, make your lower lip slightly bigger than the upper. Using the lip puffiness sliders should be done sparingly, for best results look at them from the side view and ensure that you can still see where the edges of the lips are in the silhouette. Other adjustmenets like the cleft is up to personal preference, but reference images should be used. Try to find people that are not using makeup when looking, as that can affect greatly how the lips look.
Human ears extend roughly from the eyebrow line to the bottom of the nose, and they do not stick out too far. As a rough guideline, you shouldn't be able to make out the detail of the inside of the ear while looking from the front. Aside from that, the shape is largely up to preference. Unattached earlobes tend to look the least strange as far as the mesh goes. Again, look at photos to get an idea for the overall shape.
There are no real easy anatomical guidlines here, It's mostly a matter of how you want your face shaped. Stronger/sharper jaws make you look manly, softer, rounder jaws make you look more girly. You can reference your jaw to the rest of your face if you wish, since it was used for the other measurements. It's one of those things you'll likely be tweaking for a while. I recommend leaving overbite/underbite alone, and smoothing out the chin cleft, the cleft is something best left to the skin you're using, as it doesn't much affect the silhouette of your head from most angles. All it does is make your face look strange. Use the double-chin slider to help smooth out your jawline looking from the side. You don't want it to pull up into your neck too harshly as that just looks weird.
Things like face shear and crooked nose should be left alone unless you're trying to look derpy, your brow is best left alone, or made to look as smooth from the side view as possible. You can use a combination of cheekbone and jowls adjustment to achieve a smooth looking round-girly face that looks nice from almost any angle. It is tricky however, and may take some time.
Step 3 - Sliders/Adjustments not mentioned here:
Body Fat & Body Width:
Body Fat is usually best left at 0. Some people will add a little amount as they feel it smooths things out, but in reality these are probably best left to other adjustments. The other issue with adjusting body fat is it will quickly cause issues with some mesh items, and make other sculpted or prim items more difficult to fit.
Body Width is one you can use for achieving a particularly wide or thin look, but is best used for smaller adjustments to your overall shape. Remember however that like the height slider this does not always adjust things proportionately, so tweaks may be necessary.
Hip Height & Butt Size:
Hip Height is really a matter of preference or finding what looks right, same with Butt Size. Look at photos as always, don't make it too long or too short (Or too big or too small for the butt). There's no real guidline to this. Some people like big butts, you cannot lie.
No. Just No. All this slider will serve to do is make things like knee-length boots not fit properly, and make you look exceptionally derp. Just leave it alone.
Feel free to add some if you like, or especially if you're intending to make an overweight or pregnant shape, however this is usually best left alone for the same reason as the body fat slider. It's going to make some things difficult to fit, especially mesh jeans.
Any others are probably best left alone, or just play with them and see what they do and if they can help you achieve the look you want. Experiment. That's how you learn.
Step 4 - After you're done:
Go back and check your height. If you're off by a lot, you should probably make smaller adjustments when tweaking your shape. Fix your height with the height slider if you're too tall or too short and tweak parts if necessary. Give yourself a once over and see if there are any final adjustments to make before you consider yourself done. That's all, really. Congratulations, you've made a shape, and (hopefully) don't look terrible.
Step X - Random tips on getting the best look possible:
Tweak, tweak, tweak:
Never be afraid to go back and adjust something. The Second Life body is weird and has some odd issues. Tweaking things to get better, smoother lighting and eliminating little parts that poke out are the key things to looking good, as they're some of the first issues people will notice with an otherwise-decent shape. You'll be tweaking for a long time to come, most likely, so don't worry about it. It doesn't mean your shape is bad or done wrong. Everyone does it.
Names and numbers mean nothing:
Don't worry about what a particular shape slider says, or what number is on it. Some people think that 50% means average - it doesn't. Don't look at the name on a slider, grab it and move it around and see what it does. Stop when it looks proper, and ignore the rest.
Use the right hair:
Much like in the real world, not every hair style fits every face in Second Life. Finding a hair that frames your face well is just as important as having a good shape and a good skin. Always check a demo to see how it looks on your avatar, and if a particular style is that important to you, you can create a modified version of your shape that looks nice with it.
Don't skimp on your skin:
A good skin can really make you look your best. Places like Pink Fuel, LAQ, and Curio tend to have some of the nicer ones for girls. Expect to spend L$1000+ for something really good. If you're hard up for L$, look up the Eloh Elliot MIT-Licensed templates for a good free skin that even has AI/PSD/XCF documents available for you to modify to your heart's content.