The key to storing paint for future use is to keep air out of the can. Use these strategies:
Clean the lid. Getting paint off the lip of the can creates an airtight seal.
Layer. Place a piece of plastic wrap between the can and lid to ensure a tight seal.
Close firmly. Lay a piece of wood on top of the lid and tap it with a hammer all the way around. Never hammer the lid directly as it will warp the lid and prevent a proper seal.
Label. Write on the side of the can with a permanent marker: brand, shade, color code, where it was used (e.g., living room trim), and the date purchased.
Store. If sealing with plastic, store cans uprights. Some suggest storing paint cans upside down so the thin paint skin forms on the bottom. Storing them upright, however, you can remove the top skin so that it doesn't get mixed into the paint. Store cans in a cool place, avoiding extreme heat and freezing temperatures, such as the garage, which will ruin paint.
Flickr photo by JasonLangheine
For more cleaning tips, check out What's A Disorganized Person To Do?