For one thing, you have to know what you're looking for. Expos will usually have a floor plan of their vendors. If you know what vendors you're looking for, it will save you a lot of time waiting in line, or even the crowds.
Also, do your research. Ask the important questions. I grilled each wedding coordinator, photographer, DJ, and florist I came across. You can usually tell who's the real deal and who's not when you exactly what you want your vendors to do for you. We knew that if we went with a professional DJ, we would want them to have a lot more duties and responsibilities--in a way, function as a reception coordinator. They would need to keep the reception and entertainment moving along as planned. We found one DJ who seemed totally clueless to that concept. C'mon, we're not paying you $1000 just to play music off your iTunes! One DJ marveled at how many great questions we asked. I know a little about Photoshop, photography and retouching. When I asked one photographer of the basics of her technique and the software she uses, she kind of skirted the questions and never really answered them.
We did find some really good professionals who seemed to know what they were doing. It was really helpful to see who was out there, their rates and ask them questions. Really, it's a one stop shopping adventure.
There was a bridal gown fashion show and a selection of designer wedding gowns at discounted prices. There weren't really any dresses in my budget that I really liked, but I suspect I should have come earlier for the deals.
Another great perk at a Bridal Expo is that there are usually contests you can enter for free stuff (I entered our names for as much free stuff I could!) and deals on a variety of items. I got a card for an extra $50 off of any dress I purchase at David's Bridal. Perfect because I have another appointment with them in a couple of weeks!
One other piece of advice--set up another email account just for wedding stuff. When you register for the expo, you'll be asked to give your email address. If you don't want your primary email to explode with Wedding spam, it will be good to set up another account.