Summary
Like so many things, with crowdsourcing design what you put into it often dictates what you get out of it. If you provide designers with a vague project description, chances are you are going to wind up with designs that don’t capture what you were after.
Description
Be sure to give designers very specific feedback on a regular basis, so they know if they are on the right track. Similarly, if you think a designer is just not getting it or isn’t right for your project, let him know (politely and professionally), so he doesn’t waste his time.