Make sure you don't bring too many people with you. While its very tempting to bring a lot of people, keep in mind that everyone will have an opinion. You don't want to be confused, so keep it at a minimum.
Listen to yourself! If you think that a dress is "the" dress, go with that feeling! But at the same time, don't feel compelled to find a dress on this 1 day.
Bring your camera! It will be very helpful when you want to remind yourself of what a certain dress looked like.
Definitely try on styles that you wouldn't have ordinarily considered!
Oh, in addition to the boy shorts, have a strapless bra!
MOSTLY...HAVE FUN!!! It can be stressful but this is a big day for you!