Cleanly extract homemade candle from glass or plastic mold?
Wax and Glass have very different rates of thermal expansion. Once the liquid wax has cooled and become solid, and you don't think the glass is at a risk of shattering, I would suggest putting the wax filled mold in the freezer. The wax should just pop loose. If it doesn't a quick rinse of the glass in warm water should release the wax.

This trick also works for wax on tablecloths, napkins, etc.