Remove wax residues: the bain-marie technique.

Star by boiling water in a saucepan and then put the candle jar in the double boiler to soften the remaining wax.  Once the wax has melted, remove the har and use a knife to scrape whatever's left.  Wash it with a sponge or in the dishwasher.

The Freezer Method

Wait for the candle to cool completely and solidify. Next, place your candle jar in the freezer long enough for the wax to freeze completely.  Overnight should do the trick.  The cold will cause the wax to peel off the edges of your jar so you can easily remove it with a knife.  To finish up, wash your candle jar with a sponge or put it in the dishwasher.