I was reading through some online marketing websites yesterday and one thing I found is that contests work really well in getting people on forums engaged and active.
Before you run a contest, make sure you have a written set of rules posted somewhere.
Also, there will be a temptation to award the prize to a friend instead of the actual winner, but don't fall to that temptation. It almost always backfires.
Also, try to make the contest based on something quantifiable. If you make the contest reliant on you or your moderators judging "quality" or "style" things will be more difficult.