Be clear and focused regarding what you want your website to accomplish and who your intended audience is. Think strategically. Make sure your desginer understands your mission, goals and objectives.
Provide as much material as you can to your designer, including headlines, text, thumbnail sketches, images, and any other ideas you might have. Your designer will have additional ideas and suggestions for you, but writing things down before you turn the project over to the designer will keep your stress and design costs to a minimum. It will also speed up the design process. Make sure your plans include a detailed budget and timeline for getting the project done, too.
It sounds basic, but sometimes this is overlooked. Make sure your designer has your business card, letterhead, company brochure, copies of your current advertising and promotional material, an electronic copy of your logo, and anything else you think might be useful background. Any company style guides or branding standards manuals you have will be critical for the designer to have.
Throughout the design process, not just at the end, provide feedback to the designer. What works? What doesn't work? Does the site download quickly? Does the color scheme match the intended audience? Will the site look good and work on at least most browsers, browser versions, and platforms? Do the labels on the navigation buttons clearly describe where the user will go after clicking? Is the navigation intuitive? Does the home page attract the user to stay and browse your site? Does the page look good (and is it easily legible) at all screen widths from desktop computers to smartphones.