It nearly always happens that when you finish a painting project, there is a paint leftover. This isn’t such a bad thing, though, since it is not unlikely you’ll want a bit more for touchups or matching a detail somewhere. The key to storing leftover paint is to limit air exposure since evaporation is how paint dries and becomes unusable.
In order to keep your leftover paint from drying, always cover the can unless you are actively using it. Be careful not to get the rim full of paint, as that can prevent it from sealing properly. Once you have used at least a quarter of the can, there begins to be a good amount of air inside it, even when closed. Use a clean plastic bag and lay it flat on the open can, then gently and evenly push it down until it contacts the paint. Now put on the lid over the plastic. This way there is a barrier between the surface of the paint and the air in the can.
Another option, especially if there is not much paint left in the can, is to pour it into a different plastic container. Anything with a lid will do — Tupperware, water bottles, yogurt containers… just don’t confuse the leftover paint with your leftover dinner and stick it in the microwave! Use the container that will hold the paint adequately but minimize the amount of air. One advantage to a water bottle is that you can squeeze out the air before screwing the lid on, thus eliminating all air that would ruin your paint!