To pull off your dream wedding, you need a planner who shares your vision. Look at candidates' portfolios, and compare those weddings to the one you want. If your vision is significantly different, move on to the next interview. Keep in mind that someone who primarily oversees traditional church weddings may not be able to successfully plan your beach vows.
It also helps if the planner has worked with the florists, caterers and other vendors you're considering. Not only will the planner be able to offer feedback on their services, but he or she may also have the clout to score some discounts for you. Avoid planners who try to discourage you from using specific vendors without a solid reason -- they may be trying to steer you toward other companies that them offer kickbacks for new business.