Keep a list on paper, of fleshed out characters. You want to document POV, voice, background, mental state, physical behavior, status, you name it. Controversial - some folks claim this actually un-frees your mind when playing, others find it helpful as part of their Bag-O-Tricks. Your mileage may very.
If you don`t like getting smooshed into other, stickier, smellier people, you`re in the wrong line of work -- Frank Oz