2. Let the wax melt all the way across.
Once your candle is lit, don't blow it out until the top layer of wax has melted all the way across. This might take several hours — so don't set out to burn a candle at all unless you've got time to kill.
Whenever you fail to achieve full melt, you're contributing to a process that many refer to as tunnelling. The wick starts to sink lower and lower, like a tunnel is forming right through the center of the candle.
Eventually, the tunnel will grow so deep that it'll be tough to light the wick at all. More importantly, all that unmelted wax on the sides represents hours of lovely fragrance and burn time you bought but won't ever get to utilize.
It takes patience, but if you melt the wax all the way across every time you burn, the surface of the candle will stay flat and the sides of the jar will stay clean, all the way down until the candle is spent.
3. Keep the flame away from moving air.
Do your best to keep your burning candle away from fans, air conditioners, open windows, or heavily trafficked areas where people walk back and forth a lot. Moving air can disturb the flame, leading to even more unsightly black marks on the glass.
Extra tip: Be wary of extra-wide candles that only have one wick. If the candle surface has just one wick and a diameter of more than 3 or 4 inches, don't buy it. One wick will never produce enough heat to melt it all the way across.
Right now, we only have one size option for our candles. The jar is 2.75" across in diameter and the wick should be sufficient to melt across the top. You never want to start burning the candle only to blow it out a few minutes later, that will cause it to start tunneling.