We used 99designs.com for our logo (www.dojolearning.com) as well as the new logo and product box for www.sitellite.org with great results. Our design was done

We used 99designs.com for our logo (www.dojolearning.com) as well as the new logo and product box for www.sitellite.org with great results. Our design was done by a separate designer though (he did 3 different styles that we offer in a handful of colours so customers can choose). The key was to provide good and prompt feedback, which resulted in improvements to entries. Communicate and be appreciative of their effort and it seems to go a long way (doesn't it always though? :). We also looked at designs from other contests and sent messages with "hey I like your designs!" to designers who were really good, and that seemed to help get a few top-notch designers onto our contests too. I've since recommended this approach to several other companies I work with, and they've been very happy with using 99designs as well now.