That's right! The proper way to store paint is upside down! To start you'll want to take a piece of plastic wrap and cover the top of your open paint can. Make a tight seal and then place the lid to the can on top. Attach securely and flip your paint can over! Viola! The air will be kept at bay in a moderate temperature environment.
If you'd like to keep a small amount of paint on hand for touch ups, simply place a marble to a pint mason jar or baby food jar, add your paint and close the lid. It will limit air in the container and will help keep things stirred for easy use. Place a label on the outside of each container so you remember what the color is, what room it belongs in and what the numbers are if you have to recreate it again down the road!