If you want to enhance or overhaul your existing site, ask for a critique. To save money on the redesign, first weed out outdated or otherwise extraneous material from your site. Don't get snowed by a slick pitchman who throws around buzzwords. Ask about the consultant's experience in each area needed. A consultant talented in one area, such as programming, may not have much experience in another, such as marketing.
Some consultants handle the entire process, including site hosting and promotion.
Get a projected time frame for completion of the project. It typically takes from several weeks to several months to build and test a site. If your designer works by the hour, ask to be alerted if the project is going over budget. Inquire about arrangements for maintaining the site. A consultant can do this for you or provide the tools and training for you to do it in-house.