I've used elance for finding designers. Though it worked out for me, it is not guaranteed that we always hook up with a good designer. I think we've to make sure that portfolio is decent and also make sure you write terms and conditions with low upfront fee/very close 1st milestone with at least 1 web page design. If you feel that it is not going in the right direction, dont hesitate to back out. I think the first few days of design activity is crucial and you need to make sure you've interacted sufficiently with the designer, before cutting the relationship.

obviously, if you find someone good, hang on to that relationship.