I highly recommend sending private messages to 200 designers on 99designs saying you like their work (best if you link to something they've done that you like) and would love for them to participate in your logo contest.
I think painstakingly sending invites and paying a lot are the two biggest factors of 99designs success.
If you have graphic design experience, check out the on-going logo design competition at 99designs for the Singularity Institute. There are still 6 days left to enter and be eligible to win the $295 prize if your design is selected. Tell your friends with graphic design experience too. There are very few submissions currently.
Note: This is a blind contest. Designers can only see their own entries. All designs will be revealed when the contest ends.
If you're interested at getting a peek at the designs, they will be online after the competition is over. This is standard practice in 99designs contests to prevent designers from contaminating each other and having all the designs drift in a certain direction.