I really liked this idea from the folk at Unclutterer today. They suggest storing the leftover paint from a room painting project in clear plastic water bottles (you just pick a bottle that's the right size to hold the paint without much air space remaining). They suggest throwing a couple of marbles in there, too, so when you need to do a touch-up, you can just shake the bottle.
One advantage I see to using a plastic bottle is the fact that if there is still a little air, you can squeeze the bottle to force the air out before you seal it closed. Also the fact that it's clear makes spotting the right paint color quick and easy.
Matt mentioned not wanting the paint that the homeowner left behind when he bought his current house, but I disagree. You can't always paint right away when you move in, nor do you always want to, before you get the feel for the house and decide how you want to decorate it. I have found it helpful in the past when a former resident left behind small amounts of touch-up paint so that I could fix up the bashes and bonks from moving furniture out and then in. When we move (and I hope we never do that again), I have always left the touch-up paint for each room in the closet of that room, clearly marked.