Every few months check and refresh your paint supplies.  Whichever paints you use acrylics, enamels, or other solvent-based paint it is a good practice to check up on your paints and to give them a good shaking.  Check your bottled paints, do they need to be refreshed with a dab of water or mineral spirits?  What about your marker\paint pens?  Then you might also want to do a spot check on your glues, putty, clay, stains, and pigments, to make sure they are in usable condition.