Seth Godin's guidelines on working with designers are so good that I've copied them here verbatim. -source
Here are four postures to consider in working with a good (or great) designer:
I know what I want. Bring your vision. Bring in your folder of typefaces, images, copy. Be very, very specific. The more you paste it up and sketch it out, the more likely you'll get exactly what you were hoping for.
I'm not sure exactly, but I know what it rhymes with. Put together a scrapbook. Find examples from other industries. Do you want your website to look like one from Apple or a direct marketing diet book site? Don't tell the designer what to do, but be really clear what you want to remind people of. Originality isn't the primary goal of design, effectiveness is.
I'm not a designer, but I understand state change. Do you want this work to increase trust? Desire? Confidence? Urgency? Who's it for? What's it for? If you can be really clear about what the work is for, then hire someone you trust and give them the freedom to find a way to cause that change to happen.
I'll know it when I see it. Please don't do this unless you have a lot of money and a lot of time (and a very patient designer). This demand for telepathy is for amateurs.