Trim your wick. It gives your candle a brighter, cleaner burn. Leaving your wick untrimmed and laden with leftover carbon leads to a duller light that's more likely to produce smoke. That smoke can also stain your jar.

A good rule of thumb is to trim your wick after every four hours of burn time. When it comes to trimming, always extinguish the flame, let the candle cool to room temperature, and then trim the wick to 1/4 of an inch. You can trim it with a scissor or spare nail clipper, but there's also this excellent 

You can get rid of that unsightly tunnel by folding a piece of aluminum foil in half, wrapping it around the candle, and then folding it up over the flame like a tent (leaving a hole for it to get oxygen). Keep the foil on for about 30 minutes, and all of that built-up wax should be melted off the sides.