In terms of preparation, finding the perfect dress is kind of like studying: The earlier you start, the greater your chances of success.
Give yourself two to three months for a nice, low-stress buffer. That's plenty of time to visit multiple stores, see what's out there, compare, get input, have any necessary alterations done and then style with shoes and accessories.
Budget, too, should be addressed here. Going into the search with no sense of an upper limit can result in the devastating discovery that your perfect dress will not, in fact, be coming home with you. If it's out of your price range, don't try it on; just admire, take note and keep it in mind as you comb the racks. Creating an eBay search for that dress or something very much like it might even result in a huge, happy score.
Which brings us to the less-beaten path: Mall stores and boutiques are not the only places where fabulous dresses live. Not only auction sites but also Etsy and consignment stores might allow you to pick up a dress you couldn't afford in a traditional retail setting -- and it might even be brand-new.
Finally, what may be the most crucial element in the plan: Find out if your school has a prom dress code. Wearing the perfect dress will just be sad if you can't get in.
Now, on to the good stuff: What's your color?