The next time you repaint a room in your home consider painting the inside of your light switch cover with same paint you use on the walls; if you ever need more of that colour you can remove the cover and take it in to be matched.
You can also keep track of your room colours by applying the label from your paint can or just writing with a Sharpie on a piece of painter's tape the brand, colour name, and swatch number, as shown in the photo above and mentioned before by Lifehacker. With many designer paint pallets switching regularly I think colour matching the light switch cover has your best chance of success but there's no reason you can't use both techniques to make sure.
This tip comes Pinterest user Harriet von Grinsven Cameron and the image is from Reinman Publications. From the URL it appears to have been from their Taste of Home magazine, but I couldn't find the article on that magazine's site. If anyone has a better link to the source please let me know and I'll update the links/sources.
Never again forget your room's paint colour choice [Reinman Publications via Pinterest]