It isn't always easy.  The key here is to trim the wick just enough to eliminate the smoking, yet have it long enough to maintain an adequate size flame to ensure that the candle will burn clean out of the jar.

Also, watch when you light the candle!  Often, some of the soot is formed not by the burning, but by the match or lighter used to light the wick, especially when the candle is deep inside the jar!  Try long fireplace matches or long handle lighters!