I used 99designs recently for a logo, and the experience would have been poor except for one huge tip that one of the designers gave me.

As the site is very busy, the best designers might be ignoring your contest. What you can do is visit lots of other contests, and pick out the artists whose submissions you really like. Invite them personally to submit in your contest.

The moment I started inviting the best artists I could find, my contest took off and the logo I ended up with is smokin' hot.