Seagoville , 75159

Creative Carpentry gave some GREAT tips that most people never think to do :) I have a couple things I'd like to add that may also help:
~Many paint companies have begun offering their latex / water-based paints in plastic gallons rather than metal, for example Behr from Home Depot. In this case, you won't need to worry about the metal of the can harming the paint.
~NEVER store paint on your basement and/or concrete floor/slab. It WILL ruin the integrity of the paint, rendering it useless, so always put it on a shelf that's at least 6" off the surface (if storing in your basement and/or above concrete) or simply store it on a floor other than your basement/concrete slab, like a spare closet or garage shelf.
~NEVER expose the paint to freezing temperatures. Once even a little of the paint freezes, the chemical integrity of the paint is obliterated and is not worth attempting to revive.
~If you find yourself with a small amount left over and you just want to get rid of it, there are powders that you pour into the paint and after stirring for a bit, the paint becomes one mass that can be disposed of appropriately and the container recycled (depending on where you live).