1) Make sure you burn the candle until there is an even pool of melted wax across the top of the candle. This will also ensure you get the best scent throw.
2) Before lighting your candle again make sure you trim the wick to 1/4 of an inch. If the wick is too long it can produce a large flame making your candle burn faster.
3) Keep your candles away from breezy/drafty areas. The breeze can cause your candle to burn unevenly which will ultimately effect the life of your candle.
4) When not in use store your candles in a cool place away from heat (stove/fireplace). This will cause your candle to melt when you're not even burning it.