I always tell my brides to bring a pair of heels the approximate height you'll wear on your wedding day when you try on the dresses. It gives the dress a different look and it will tell you whether you want to wear that size heel all day long. You may have your heart set on a 3.5" heel, but the reality of wearing it all day will set in while you are trying on gowns.
Dress appropriately. Wear something easy to put on and take off.
Wear the proper undergarments. If you have a strapless bra, bring it along. You may or may not need it with the dress, but it will give the dresses the look the designer intended.
Wear underwear. I am so tired of hearing "I forgot to put on underwear". Yeah, right, you didn't have any clean underwear! ;) You'll have to stand in a dressing room with someone helping you dress because trying on wedding gowns is not easy to do yourself. It is uncomfortable for you and for us to see you tush. Pack some underwear, and remember, if you wear a thong, we still see your tush.
If you are going to wear your hair up on your wedding day, bring a scrunchie to pull up your hair. It will give you a different take on the neckline of the dress and the look you are after.
Bathe. I know this sounds silly, but you are trying on white or ivory dresses and you should be clean. Don't shop for dresses after a workout. Shop after you bathe. Also, don't shop right after you've tanned and used a lot of tanning lotions or spray tanned. It will transfer to the dress or to your personal wedding dress.
And above all, bring a positive attitude and an open mind. Open yourself up to trying on different styles. Most brides buy something totally different from their original idea. And above all, have fun shopping!