I can't recommend these two sites enough. Put up a reward and have a huge community compete to pitch logos at you, and give feedback to have the ones you like developed further till you pick a winner. Much better then working with a single designer that gets the shits if you don't jump at their first attempt, especially if you don't have a very specific concept in mind.
99 designs = higher budget and quality, but hatchwise is good too
http://99designs.com.au/
http://www.hatchwise.com/