FOR AN EVEN BURN
When you light your candle for the first time, let it burn for long enough for the entire surface to melt to the edge of the glass. This can take a few hours, but it will avoid the candle 'tunnelling', which is when the wax melts near the wick but not at the edges and leads to an uneven burn.
Make sure it's on a flat (as well as stable and fireproof) surface.
Keep the wick in the middle of the vessel. It can be gently readjusted when the flame has been snuffed but the wax is still molten. Once you've done this, allow the candle to solidify before relighting.
Similarly, avoid nearby drafts, vents, fans or air currents that may disrupt the flame and/or the wick, leading to an uneven distribution of heat. (Although, do remember that the room will require some ventilation.)
Always remove any wick debris.
TO PRESERVE FRAGRANCE
Do not burn your candle for more than three hours at a time.
Trim the wick to less than a quarter of an inch to avoid black smoke and residue on the glass vessel which may interfere with the fragrance being released. This can be done with special wick trimmers or regular scissors.