Plan to have your wedding dress altered. If you are a fairly constant size you may get it altered up to a month before the wedding but for brides that may be prone to losing or gaining weight with stress alternations may be needed closer to the wedding date to get just the right fit.
Never buy a wedding dress that you are planning to "diet into". Not only does this place additional stress on the bride to loose weight but it often makes alterations very difficult for the seamstress if you don't lose the weight you thought you could.