This came from getorganized.com. When you're painting a room, dip the stir stick into the paint and let it dry. Then write the color, brand of paint, sheen, and room on the end. Store all the paint sticks for each room together in a coffee can near your paint storage.
That way, whenever you need to match the color or find a complementary one, you just have to grab the stick and take it along. Also placing the leftover paint in a small, clear jar that is labeled with the room name will also help you save of space.