What I did was set up a logo contest for $400, then personally messaged 30-40 designers whose portfolios I liked on the site to ask them to participate. Once I saw enough good stuff, I guaranteed the contest (meaning I would definitely pick a winner), and asked people for modifications. That part is key; you want to find out not just which piece of design you like, but who is a good person to work with.
Moving forward, I actually ended up working with the person who won the contest AND another person who was very good, but just happened to be the wrong person for this particular design. Once they've done good work for you through 99designs, there is no reason you can't work directly together on future projects. I suspect that's part of the reason designers use the site (although many of them are also from countries where the dollar is highly valuable).