It occurred to me when I was having a conversation with a blogger who shall not be named, that a post about re-purposing candle jars without mentioning how to clean said candle jars is pretty useless.
This post has no pictures. I assume you all know what a candle jar, kettle, knife, and dishwasher look like. If not, I can't help you.
When the candle has burnt down as far as it can go, flush out the bottom with some boiling water. This will melt the tiny bit of wax left and will dislodge the metal wick holder (if not, give it a bit of a nudge with a knife handle or just anything really). If the jar has black soot in it, just use some warm water to wipe down the inside edges. Pop the jar in the dishwasher and it will come out clean and shiny.