A little hack I picked up years ago that makes life simpler. Fewer rusty, dusty, hard-to-open cans stored in the basement. Keep a couple of throw-away sponge brushes handy, and you can do touchups in a flash with minimal clean up.
Save those empty sports bottles to store your leftover paint. I like these Gatorade ones, which have a really easy to open/close cap.
Use a funnel to fill ’em up.
Drop in a couple of marbles–that way, you can just shake the bottles to mix the paint up when you need it.
Label them carefully. I like to put them in a reused six pack carton, so they are easy to stow (they won’t tip over) and tote.
When you need to do a touch up, just shake the bottle and pour a bit of paint out on a paper plate, a piece of cardboard, or other makeshift paint palette.