The first burn matters most
Let the first burn run about three hours, long enough for the melted pool to reach the edge of the glass. Wax has a memory: if the first burn tunnels, every burn after it will too.
Trim the wick
To 5mm before every burn. An untrimmed wick smokes, mushrooms and burns through the candle faster.
Stop at four hours
Any longer and the vessel gets hot enough to be uncomfortable to move.
Keep it out of a draught
A draught pulls the flame sideways and blackens the glass.