Trim the wick: Before lighting a candle after each burn, trim the wick. This helps flame from getting too large & causing the wax to melt too fast.
Burn candle for the appropriate amount of time: First burn, let the candle burn until the wax has melted all the way to the edge of the container. This will help prevent tunnelling, where the candle burns down the middle and leaves wax on the sides. After that, try not to burn candle more than 4 hours at a time.
Keep candle away from drafts: Drafts can cause candles to flame, flicker and smoke, which can make it burn unevenly.
Keep candles away from heat and sunlight, this can cause them to melt.
