Load a Dixie Cup Dispenser ($4 at a grocery store) with 3- and 5-oz. paper cups and keep it handy for all kinds of jobs. Mix epoxy, measure liquids for finishes, hold glue, keep track of parts from disassemblies and hold paint for touchup jobs.
You’ll get so used to grabbing a cup that you’ll wonder how you ever worked without them. Thanks to Dan Allmon for this tip.