I'm a programmer by night / business type by day. I have a website soon to be released in beta and I hired a designer I trusted (and could afford) from a previous job. She worked wonders for the project -- seeing the new design also reinvigorated me.
A few tips: Hire a friend if possible (you'll probably get a friend-rate), if not work by referral. You want someone you can sit with face to face. Avoid 99Designs and the like if you can. Pay your invoices immediately and you'll find the designer will likely continue to be super helpful. Also, get an hourly rate up front since you'll probably find things that need to be adjusted. Lastly, I wouldn't use the designer for HTML, just have them hand off PSDs, the markup tends to be not so good. Insist on PSDs, not flattened images.
By the way, I'm happy to give a refferal for a trusted friend. Just email me at juliuss at gmail.