Once you light your candle for the first time, always let the top layer of wax burn until it is melted all the way across the jar. This will prevent tunnelling. It may take a good hour or two, so don't light a new candle unless you have some time to burn.
I recommend always keeping your wicked trimmed to 1/8-1/4" in length. Keeping your wicks trimmed will keep the flame controlled and give you a much cleaner and brighter burn. Excessively long wicks end up leaving those nasty smoke stains inside your jars. Yuck.
Keep your flame away from fans and moving air. Open windows, air conditioners, or even rooms with heavy traffic can disturb the flame leading to uneven burning and ugly black marks on the jar.
Try not to let a candle burn for more than four hours. Anytime you burn for more than four hours you run the risk of the wick 'mushrooming' or even floating to the side which will lead to an uneven burn.
Seriously. I thought snuffers were another marketing ploy, but when you blow out a flame, it actually creates and spreads those little annoying black particles that get stuck in your wax. You don’t have to buy a snuffer though. Just use your candle lid, and it’ll smother the flame in the same way. Plus, a top keeps dust and dirt out. Best to let the candle cool completely before touching or moving it as well.