Leftover wax in a used candle jar is notoriously tricky to clean out. Throwing the candle in a freezer to loosen up the residue may just do the trick.
J. Angel posted this tip on Snapguide, explaining that the freezer will cause the wax to contract from the sides of the jar, making it much easier to scrape out with a dull knife. Once most of the wax is out, a few quick swabs with baby oil or vinegar should clear up any residue, leaving you with a perfectly good jar for countertop storage.
Remove Residual Candle Wax From Candle Jars [Snapguide]