I don't let my jar candle burn for very long; mostly just long enough to seal the end of cut ribbons. Because the wax doesn't have a chance to volatilize much, the wicks get lower and lower. I tried running the molten wax along the edge to get it away from the wick, but that just made deep holes around the wicks. Then I had an experiment that needed flame for a little longer and the melted wax enveloped the wick and put out the flame! Oops. So to fix it, I buried birthday candles down into the soft wax. Not only did I have flame again, but the wick from the birthday candle could draw wax from the jar candle when it was low enough. Two new wicks rather than throwing out the candle! And quicker than melting the wax and re-pouring the candle.

Birthday candles buried in the melted wax of a jar candle to replace the wicks that were too short