Underestimating your timeline
Many brides will go shopping without knowing how long it actually takes to order a wedding gown and then alter it. On average, it takes 4-6 months to order a dress and then you need 2 months to alter the dress. It's always best to shop and order 9-12 months before your wedding so you don't run into problems ordering or altering. If you have a shorter timeline, your options may be limited and rush fees may be added in the ordering process or the altering process.
Not sticking to your budget
I have seen this happen to way too many brides. When you come in without a budget (or when you're not open with your stylist about your budget) that can cause appointment disaster! You will have a very hard time finding the perfect dress because you will end up falling in love with a gown that is more than you realistically want to spend. So, be sure to set a realistic budget for your gown before you go shopping, so your stylist can help you find what you're looking for in the price range that's right for you!
Bringing too many guests to your appointment
I know that dress shopping is such a fun and special experience for you and you don't want to leave anyone out of the whole process, but bringing too many people can really make it harder to find the right dress. More people in the group means more opinions that will either sway your opinion or fight with your opinion and your opinion is the most important. Dress shopping can be a very emotional experience, so when you add too many people then you are more likely to lose your own vision.
Buying a dress you don't love
Always remember to trust your gut and don't let anyone else's opinions change your mind. You don't want to be in a position buying a dress that you like and your party loves because then you're stuck with a gown you don't love and can't return! I have seen too many brides get talked into buying a gown they don't love because their mom loves it, or their sister loves it, or their bridesmaids love it. It's not their wedding, it's YOUR wedding! Get the gown that you love and don't let anyone else's opinions take that away from you!
Shopping after you find the one
Once you say yes to a gown, then the most important thing to do is STOP SHOPPING. It causes unnecessary stress and turmoil for you because you already found the one! Be sure to take plenty of pictures and videos of your gown when you order it so that you can revisit it whenever you miss it.