I learned a valuable lesson when we moved out of our last house. After painting the walls, I kept the leftover paint "just in case." After 6 years of sitting in the garage, the paint dried up. So naturally, one day when we were cleaning the garage, I looked inside each can and figured, "Why have I been keeping these cans? There's barely any paint in here!" (*toss*)
When we moved out, I pulled pictures off the walls and realized I needed to touch up some of the paint in almost every room of the house. (each painted a different color) I quickly remembered throwing the cans away and was kicking myself, wondering how I'd get the hardware store to match a color that I couldn't show them! I actually had to take pieces of the dry wall in with me...it was a disaster.
So when we painted the kids' rooms and my craft room when we moved into this house, I wised up. For two of the rooms, we used every drop of paint in the can, so I just kept the lid in a Ziplock bag. That way, when I need touchup paint, I can bring the lid into Lowe's and they can mix the formula on the tag on the lid. I felt like a genius for coming up with that!
This came in VERY handy yesterday when the painters came to paint over the dry wall repairs they did the day before. I handed them the Ziplocks with the paint lids and they were able to match the color exactly. One of my neighbors had such a problem matching a color in his house that the painters had to re-paint the whole room in a similar, but different shade of green. They did it, but it was a lot of unnecessary work.
Also, if there's just a LITTLE paint left (that will likely dry up soon) you can transfer it to a smaller container (I used an empty baby food jar) for smaller touch ups.
That's my big tip of the day. Save the paint lid, not the whole can (unless there's a bunch of paint left in it, of course). It totally worked for me!