A few 255 item stack methods

Red/Blue:
1) Old man glitch: Get 129 items in slot 6 first by encountering Missingno. with 1 item in slot 6 (because Missingno. adds 128 to the sixth item), toss 2 then encounter Missingno. again. Alternatively, with items like Potion after it becomes 129 you can use it twice within the battle and catch Missingno. to do two duplications in one).
2) Trainer-Fly glitch: Use one of these yields http://puu.sh/257S or Ditto glitch with a Special stat corresponding to a Missingno. (Fossil and Ghost Missingno. work too) then do the same steps as above to get 255 items.
3) CoolTrainer glitch: In Diglett's Cave, view the "-" move in battle (obtainable by swapping move 1 with move 2-4 on transformed) Ditto or glitch Pokémon that learn it when they already have 4 moves http://glitchcity.info/wiki/index.php/-_(move)#In Pokémon Red/Blue) and hope for corruption of the enemy Pokémon (music should fade out) after opening the Pokémon or items menu within battle (this is important). Then catch the enemy, it should have become Missingno. and your sixth item should be duplicated. Toss 2 of it then repeat, you may have to fix the messed up graphics first by viewing the summary of a non-glitch Pokémon otherwise you may get Horsea the next time.
4) Cable Club escape glitch - do it with someone who has a Missingno. https://www.youtube.com/watch?v=JgVUZPTt21E

Yellow:
1) Trainer-Fly can be done, recommended Specials of 182-184 for Yellow Missingno. guaranteed to not freeze the game.
2) CoolTrainer can also be done to encounter Missingno. in the same way as Red/Blue after you obtain it except you can do it outside of Diglett's Cave (theoretically anywhere with grass/encounters), but you need to get "-" as the first move in some way. The easiest way is to trade a CoolTrainer Ditto from Red/Blue to Yellow.

But you can also use a ♀ . (hex:C1, http://bulbapedia.bulbagarden.net/wiki/%E2%99%80_), hope that you don't get a freeze, then level it up to level 16 or 22 where it will learn the CoolTrainer move. You can get one through Ditto glitch but the game might freeze. It didn't for me though, mine was encountered on Route 6 if that made a difference.

Make sure to level it up in a safe spot to avoid long move name freezes, such as here http://i.minus.com/ieB2Tly27VDEn.png. A more complicated way to get - as the first move might be a variation of the box data shift glitch (example: https://www.youtube.com/watch?v=tQ9JAJcofrs) after enough shifts to bring an empty move to the top with an experience byte as the FF value (I just had some trouble where the high experience due to FF bytes being copied up made the game lock-up after withdrawing the Pokémon though.
3) Cable Club escape glitch (with stable Missingno.)

Other ideas:
Moves of Pokémon beyond slot 6 having - as the first move?
Map FE (though this completely breaks things).