Latex – Remove the lid and air dry the paint in the can. To speed up the process, stir in an absorbent material such as kitty litter or sawdust. For large amounts, line a sturdy box with a plastic bag. Fill with kitty litter or sawdust and pour in the leftover paint. Put in the trash once dry. Wet paint put in the trash can make a terrible mess when the garbage truck compacts the cans and the paint is released. This is the main reason municipalities require paints to be dry or solidified before disposal.
Oil-based– take to Household Hazardous Waste collection.