The dresses do run small, so don’t freak out if you don’t fit into the samples correctly or if they tell you you wear a size 4 sizes larger than normal. They can do a lot of magic with dress clips to give you a good idea of how the dress will fit.
You might want to bring a lightweight robe with you. A lot of dress shops provide robes in the fitting rooms so you can have some modesty while waiting for them to bring you a dress or so you can pop out of the fitting rooms to look around the floor. But in my experience, the robes can be skimpy. I went to some shops where the provided robe wouldn’t even wrap all the way around my boobs!
Wear nude colored undergarments so you don’t get distracted by a pair of black or red knickers showing. And while a lot of women wind up going braless under the dress, I also think it’s a good idea to bring a couple of nude colored bras with you in different styles.. strapless, convertible/halter, etc. Maybe some spanx if you think you might wear them, too.
If you usually wear makeup, definitely make sure you’re wearing some, and have your hair looking nice. It’s easier to picture how the dress will look if you feel good and confident about how you look.
I also suggest bringing a pair of shoes similar in height to what you expect you will wear. My first dress shopping appointment, I forgot to bring a pair with me, and all the sample shoes they had available were 2 sizes smaller than I wear. It was really uncomfortable, cramming my feet into those shoes!
Remember that very few of us have the emotional, crying, Say Yes to the Dress moment. You’re probably not going to hear angels singing once you find your dress.
If the store allows it, make sure you get photos. I think it’s really important to be mindful of how a dress photographs, given how often you’ll have your picture taken wearing it! Sometimes a dress looks lovely in person but photographs looking really strange, or you might realize you like the way the dress photographs better in ivory or champagne than white, or vice versa.
Be open minded about style. I never would have considered a form fitting sheath/column dress, but I’m glad they encouraged me to try one on; it looked so much better than I would have imagined!
Remember that alterations are going to be much more expensive on more intricate dresses.
And don’t forget about things like how easy it is to maneuver in a dress, sit down, visit the bathroom, etc.