Paint is the best place to start and of course you really can't go wrong with white!
I worked in a program where we set up houses to be sold and a few little 'tricks' that we would use on homes to make them look bigger/better. . .
Pull all furniture 4-6 inches from whatever wall it is against. This makes the room look larger.
Houseplants!!! Take away any knicknacks you have and put a houseplant there. Minimum of 3 plants per room. It sounds wacky but I dare you to just TRY it in one room - you'll see.