I totally agree with Richard Adams points when you want to choose a good designer:

1- portfolio
2- customer service
3- speed.

then when you fount your favorite designer, stick to him/her, this way you will understand each other tastes better for future projects and designs.

I have recently used graphics genie for my latest project, their speed and customer service are great and best of all they are very creative in design depending the subject of your service.

For small works I hire people from rentacoder.

Behzad