Keep the wick trimmed to 1/4 inch at all times. It is suggested to trim the wick every 4 hours of burn time. When wick trimming, you should always extinguish the flame, let the candle come to room temperature, and trim the wick before relighting. I simply trim my wick before lighting each time.
Trimmed wicks make for a clean burn.
Trimmed wicks make the flame look clean and bright. Untrimmed wicks are a lot more likely to take on a weird mushroom-esque shape that dulls and obscures the flame.