It is a great idea to store some left over paint from freshly painted walls for touch up down the road. The key to this is to store it in a place that the paint will remain fresh and ready when you need it.
Always label your paints by room for easier touch up when you need it. Do not write on the sticker from the paint store. You may need more down the road. You do not want to accidentally cover up the formula they may need to duplicate your color years down the road.
Store your paint in the smallest container possible. This will prevent skinning (The film formed on top of wet paint remaining in the can). You may also want to take plastic wrap and place it on top of the wet paint before pounding on the lid. This will prevent air from forming a skin.
When storing metal paint cans do not leave them on the floor of your basement or garage. The moisture in the concrete will rust the can, leaving you with an unsightly ring on your floor. Even if you have a coated floor the colder concrete floor and the cool metal of the can will cause moisture to condense and eventually rust all the way through.
Always store your paint in a controlled environment. You do not want latex paint to freeze. When latex paint freezes it causes the water to separate from the resins and pigments. If latex paint does freeze you will know it because it will look like cottage cheese.
When stored properly your latex paint will last for years.
For more information on proper paint storage or if you want to know if that paint from years ago is still good, come see your local the paint experts at Shelby Paint & Decorating.