Everybody knows about dipping plastic figures in boiling water, straightening the bent bits (arms, body, weapons, etc.), followed by a dunk in ice water. Frankly, I know it works, but it can be a real hassle, not to mention a bit dangerous for receiving severe burns!
I recently bought some plastsic Fire Giant figures for the Pathfinder RPG game. I also bought a bag of vinyl-like plastic skeletons (54mm?), for Giant Skeleton figures for my AD&D games, as well as my BattleSystem games (AD&D mass combat rules). Most of the figures were deformed, being packed tightly into boxes for shipping.
I reasoned that if they became 'permanently' bent due to being held in deformed positions for days at a time, then they could be straightened in the same manner. So I took some spring-type clothes pins I had in a bag in my crafting room, and I clipped them on the bent swords of the Fire Giant figures, which held the blades straight (any pin which will hold the bent bits in proper shape, will work, really). I clipped them like this for several days, and they held their straight shape thereafter. I tried the same concept on the vinyl-like skeletons, even though they were in much worse contortions. It seems to work! The only thing required, is patience. It may take days, or even a couple of weeks, but it does seem to work -- and no burns. Cheers!