I discovered the other day that candlewax microwaves well. This is probably obvious, but compared to putting glasses of wax into pans of boiling water this does make reusing old candles much easier.
I scooped up a few old candle leftovers, broke them up into a glass, nuked it for a minute or so and dropped in my replacement wick (held up with a paperclip, tied down to a 1" nail):
One error I sometimes make: Always dip the whole wick in wax for easier/stronger starting off with the new candle.
Eco-scrooge strikes again!