The First Burn:
Candles have a memory from the first burn. Make sure the first burn lasts long enough so the entire top layer of wax is melted all the way to the edge on the first go. Otherwise, your candle may create a 'memory ring' where the burn will continue tunnelling down the middle around the wick for the rest of its life.
A Bit Of Grooming Goes A Long Way:
To keep a steady flame and to prevent the wick ends from falling into the wax, keep the wick trimmed to about 5mm before lighting. Dont play with fire and make sure the flame is out and the wick is cooled before trimming.
Keep It Company and Know When To Say Goodbye:
Never leave your burning candle unattended, and try not to let it burn for more that 2-3 hours at a time. Once the wax reaches the halfway point, reduce your burn time to 1-1.5 hours sessions to prevent the jars from overheating. Watch out for the surface too, as the bottom of the jar can get quite hot once the candle burns down. Keep your candle away from strong drafts and extinguish your flame safely each time.
For your safety, it's time to stop burning your candle when the wax is around 1-1.5cm from the bottom of the jar. But never fear,, you can wash your vessel out with hot soapy water and you have a pen holder, pot plant holder etc...