Posing

I've always had huge trouble with posing, there's no good advice out there at all. The basic concept is

>make tense pose look relaxed

You always have to check what the photographer is looking for tho, for example people used to ask me to lower the angle of my face a bit, so I always do that on photo sessions, but on my last shoot the photographer kept asking me to do the opposite. It felt a bit counterintuitive at first.
In summary you have to try and not get over your head, just have fun, feel your body move and try a lot of different things (even really dumb things), people will tell you what's working and what's not.

Some tips:

>Loosen up your jaw, and try to avoid having your mouth totally shut. A good tip is blowing some air through your lips.
>Pose with your fingers, don't let them dangle like loose spaghetti. Bend one, twist them, make them touch, etc.
>Make sure your body isn't pressed against itself, your arm for example, even if it's against your chest or something, leave a little space in between (if you press all the fat will be pushed forward and your arm will be twice as large, it'll look awkward)
>Same with your head, try and 'pop' it out, like a turtle. It's weird but it helps define your features against the rest of your body, plus slim neck/no skin wrapping
>Add intensity to your eyes, 'smize', make them 'sharp'. A good tip is to sometimes try and look 'beyond' the camera, try and focus your sight through it.
>Practice with a mirror, look for some interesting poses onlne and then copy them, in time muscle memory will do the work for you.

In essence just a lot of practice, trial and error, having talented people give you good directions. Also, most of the times you won't like the end result, don't be discouraged by this, it doesn't mean they are bad pictures, just that you want to get better at it.