Believe it or not, proper paint storage begins from the moment you open the can! It's important not to bend the lid or rim in order to maintain the can's integrity. Invest in a paint can opener, available for about a buck at most paint and hardware stores.
The lid well (the groove between the can's edge and opening) often gets gunked up with paint, which inevitably dries and prevents the lid from closing properly. To keep the well clean, always wipe the rim after pouring with a damp cloth or your brush.
And never wipe your brush on the edge of the can! Instead, after you dip your brush in the paint, give it two quick taps on the inside of the can. That way, your rim stays clean, and you avoid wiping off all of the paint you just applied.
To prevent can distortion, never take your hammer to the lid. To close the can, use a rubber mallet and tap your way around its circumference. If you don't have a mallet, place a block of scrap wood on top of the lid and hit that with your hammer instead.
When it comes to storing paint, it is important to prevent air from seeping into the can. The fuller the can, the less the air can get in. Therefore, consider decanting smaller quantities of paint (for touch-ups down the road) into smaller plastic storage containers or recycled water bottles. Drop a few marbles in to help with mixing when it comes time to use the paint again.
Larger quantities of paint can be safely stored right inside the can. To keep the air out, drape a layer of plastic wrap over the paint and another over the top of the can before sealing it shut.
Paint must be stored in a cool (not cold), dry place. Keep cans up off the floors (especially concrete ones) to prevent rust. Utility closets and basement storage rooms are ideal.