I keep all the caps from bottled water. Perfect little vessels for small amounts of paints when detailing. A separate cap for each color and when you're done -> clean up is no more difficult than tossing in the trash. I have so many (I think they multiply at night) I should already have enough for the rest of my life.
I also keep no longer useful CDs -> great pallets for daubing out globs of gel type CNA glue (for use with toothpicks, etc.) or mixing epoxy. A CD goes a long way and when every square inch is covered with hardened glue globs, it goes the same route as the used bottle caps.
Another thing I do with expendable CDs (yes, I seem to have a lifetime supply of these, too) is to use them for spray painting. Loop some masking tape on the CD, stick the parts on, spray away. Rinse and repeat. Reusable for an amazing number of model assemblies.
And, larger parts are masking tape-looped onto the cardboard backing sheets from paper tablets. They last for a long time, and when you're done with them . . . well, round-file them. Another big $ investment in tools, huh?