What to wear for Walter: Glasses, a yellow hazmat suit and mask. Oh, and you'll have to shave your head and grow a goatee.
What to wear for Jesse: Same as above. Less facial hair; a five-o'clock shadow will do. Add some tattoos for bonus points, and carry around some blue "meth."
How to act: If you're Jesse, alternate between being totally cool and incredibly paranoid, and say "bitch" a lot. If you're Walt, act quietly intimidating, and if someone asks what you are, respond "Heisenberg."