Sacha Greif provides some solid advice for how to find and select a designer. This is something that many startups hit up against at some point in their life, since most startup founders are not professional designers - and, at the end of the day, there is a clear qualitative jump between what you can throw together yourself in a pinch and what a seasoned professional can produce.
Sacha's tips focus on finding freelancers, and cover where to look for designers, how much you should pay, how to pick the right designer, how to select a designer that has the skills you need, the use of multiple designers, and how to get the designer started.
Very much worth a read if you're thinking of hiring a designer for some aspect of your startup.