The key to candle life is a well-maintained wick. For a clean burn, you need to keep the wick trimmed by cutting off the black blob that forms at the top after you extinguish it.
Use this lovely brass Wick Trimmer to trim your wicks to 1/4" before each lighting for an even and clean burn.
The piece you cut off is held by the trimmer so you can dispose of it properly and avoid getting soot everywhere.
While we're at it, another pro tip is to burn the candle all the way across the top each time you light it, to avoid having the wick tunnel down inside the wax.