My experience with my first design contest (for a logo) was so good that I am challenged to find any suggestions.
Peace Football was delighted with the quality of logos that we got from our 99designs contest - so much so that we picked two winners. We couldn't decide from all of the wonderful designs we received (quite a bit more than the estimate provided up front when we picked the level of design contest we were hosting). We would heartily recommend 99designs and plan to use the company for future design projects. It is a very well run operation with easy-to-use tools that produces fantastic results. The designers are not only talented, but very accommodating to fussy clients like us.