Pick dresses a size larger than normal. Prom dresses run at least one size smaller, just like wedding gowns. Remember to always try it on. Or at least try a similar style, if they one you want isn't in the right size. Let the salesperson help you try on the dresses. They help you zip and lace up the dresses, hang them back up and save you time shopping. Image titled Shop for a Prom Dress Step 6 Listen to the salesperson’s ideas because they know which body type the dresses look good on. They know which styles are hot and which styles have a great fit. Start shopping in January or February when the selection of prom dresses is the largest. Buy your dress a couple of months before your prom to have lots of time for alterations. Be okay with change. Almost every prom dress will need some alterations for a perfect fit. Always buy your prom shoes before your alteration appointment so the dress can hemmed to correct length.