1) You may not find the dress in the picture. And if you do, it may be shockingly expensive. This may be true especially if you live in a less populated area. Instead of getting your heart set on a particular dress by a particular designer, try to disect what it is you love about that particular dress. Is it the shape, the bodice, the lace detail? Are you drawn to embroidery, beading, or a particular neckline? Figuring out the elements you are drawn to will help your consultant find alternatives that may turn out the be "the one".
2) Do not get discouraged if the samples don't fit. Samples usually come in a size 8 or 10. The average woman is a size 14. And wedding gowns run small (if you are a size 8, you may need a size 10 or even 12 wedding gown). I am convinced it is a cruel industry joke. But even if it doesn't zip up all the way, you can usually get a pretty good idea of what the dress will look like. You'll get an idea of the neckline and how it frames your face, if the waist is flattering or not, whether or not you like lace, beading, etc.
4) You may not have the "this is it" moment. Although we all think we will, it is rare. I'd say only 1-10 brides have the eureka moment the first time they dress shop. Finding the perfect wedding gown is an emotional experience, but it is also a whirlwind. By the time you are done in a shop, you may have tried on 15 different styles (because you walk in thinking you know what you want, but may have been wrong... see #3) and narrowed it down to the perfect bodice style, neckline, skirt shape, what have you. Don't feel like you'll never find "the one" just because it doesn't happen the first day. If you have narrowed it down to the dress shape and style you should look for on the second trip out, you have made tons of progress and will more likely find "the one" on your next excursion because you know what to look for.
5) Do not purchase a gown anticipating to lose weight. We all want to look our best on our wedding day, and often this means we plan to lose that Christmas ten (or in my case, twenty). And you go girl! But don't buy your dress more than a size smaller than what you are when measured. If you lose more than a size, awesome! Your alterations person can take the dress in. If you don't, they may not be able to alter it to fit without drastically changing the look of the dress (read a big white panel where beading should be continuous). The number on the dress is a number. It says nothing about how hot you're going to look (and feel!) in it. When it fits, you're a hottie potatie. When it doesn't, you feel like a sausage. I speak from experience.
8) Bring your own strapless, waist cincher, etc. These undergarments really do help give a good foundation, which in turn makes the dresses look their best when you try them on. Often you can borrow one from the shop, but would you borrow gym shorts at the gym? Gross.