One of the absolute best ways to keep soot down when you're burning a candle is to keep the wick trimmed to about ¼".
When you first light a candle, you want to make sure that the candle is trimmed down to that ¼" length so that you don't start off with a high flame. Move your candle away from any vents, fans, or air drafts. If you find that the flame is dancing and your candle isn't in a drafty room, you'll likely need to trim the wick a bit more.
This is one of those reasons why a candle should never be left to itself. If your flame gets too high, it could really mean trouble. However, keeping your candle wick trimmed will keep your flame under control and allow a better burn with less soot.
*If you are experiencing excess soot and these suggestions are not working, discontinue use and contact us.