The first burn sets the memory
Wax has a memory. On the first burn, let it melt all the way to the edge of the vessel before you put it out, which usually takes two to four hours depending on diameter. If you snuff it early, the candle will keep re-melting to that smaller circle and tunnel down the middle, stranding wax against the glass.
This single habit is the difference between using most of a candle and using about half of it. If a candle has already tunnelled, wrapping foil around the rim to reflect heat inward can sometimes recover the edge.
Trim before every burn
Trim the wick to about a quarter inch each time before lighting. A long wick burns a larger, hotter flame that consumes wax faster, produces soot, and mushrooms at the tip.
Trimming is the cheapest way to extend burn time. It reduces the consumption rate directly, which is the figure that determines how many hours you get.
Do not burn for more than four hours
Past about four hours the vessel is hot, the melt pool is deep, and the wick starts to drift or drown. Put it out, let it cool, trim, and relight. Long sessions waste wax without adding scent.
Keep it away from moving air
A draught makes the flame flicker, which burns wax unevenly and pushes soot up the glass. Vents, open windows, and the path between rooms are all worse than they look.
Snuff, do not blow
Blowing a candle out scatters hot wax and leaves the wick smoking. A snuffer or a dipped wick puts it out cleanly and leaves the wick centred and ready for the next burn.