Fortunately, as long as a paint can is stored and sealed properly, paint does not go bad. Improper storage, however, can dry out the paint or introduce dust or other particles into the paint, leaving it tough or impossible to work with.
Properly storing paint cans is easy, and will elongate the life of your paint. You should always make sure that the lid is on securely and is closed completely; this will keep air and other particles from getting in your paint. Store the paint in a cool, dry area. Excessive heat can cause the container to expand and possibly explode, shattering paint and debris. The paint cans can be stored on top of each other, but be careful not to stack too high; they may tip over and cause injury, or you may not be able to reach them easily.
To keep an eye on how much paint you have without having to open and correctly seal each can of paint individually, keep a rubber band on the outside of the paint can, stretched around the diameter of the can. As you use paint, lower the rubber band to the level of the paint. If your paint can is getting low, it may be time to buy another reserve can. Keeping basic colors such as white and tan paints handy will make simple home repair jobs much easier; you won't need to run to the store halfway through a project. Keeping a rubber band on the outside of the can will help prolong the life of the paint inside it, as you won't constantly be opening the can to figure out how much you have.
If you open a can of paint and it is hardened, or if there is a layer of dust on top of it, you will need to buy a new can. The dust can leave particles on your wall or other surface, or can cause the paint to spread unevenly. Adding water to a can of dry paint will throw off the proportions and may cause the paint to dry irregularly or with uneven coating.
If you are buying a specific or customized blend of paint, it may be helpful to buy one extra can than your measurements require. Having an extra can will allow for easy touch-ups, or will compensate for any mistakes which may be made. If the paint is customized, you may not be able to have an exact shade produced again. Having a spare can around will prevent you from any last minute errands.
The longer you can keep a can of paint sealed, the longer the life of the paint will be. Keep cans of the same color paint sealed until you have used up the remainder of the previous can.