The industry is only now defining what a good web designer is. If you already have a site, you can improve some design elements with minimal cost, others will need a complete overhaul. Whatever you do, find out what works and what doesn't on a website. Then, be choosy about the designer. Just remember that since the Web is still so new, there are some Web designers who actually do not really understand the Web itself or how to design for your customers.
Visitors leave if pages load like molasses.
Maintain your links. Visitors get annoyed with misspellings, bad grammar, and inaccuracies, and make snap judgments about your site. Lose the Bells and Whistles. Visitors are distracted by blinking animations, which tend to drive them away rather then captivate them. Exaggerated efforts to impress usually backfire.