If you are one of the lucky few who get a namebadge, plan to get there a little early or stay after your signing and stroll around and look at all the stuff. It's really an amazing thing.
Here are some general etiquette rules for the convention:
1. Don't go from booth to booth asking for free samples or catalogs. Those are to impress and entice the buyers. And they are expensive. Some vendors can't afford to give freebies to everyone, but they're too nice to say no. Other vendors believe that if they give everyone a freebie, it's all good marketing because you'll read the book and tell people about it. So if a vendor offers you a freebie, say, "I'm not a buyer; I'm an author." If they still insist, then go ahead and take it if you want it. And if you do accept a freebie, express your thanks appropriately, by READING IT! Loaning it out. Giving it a good review on every blog and forum you have access to.
2. If you're looking for a new publisher or distributor, this is probably not the best time to approach them. The focus is to sell product to the bookstores and that is the vendor's priority. Please respect that.
3. If the vendor you really want to talk to happens to be sitting in their booth, staring into space because there's been a lull in traffic, then you may gently and politely approach them and introduce yourself. But if they aren't energetically and enthusiastically engaging you in conversation, take a hint. And the minute another person enters the booth, excuse yourself and leave. You won't make a good impression if you cause the vendor to lose a sale.
4. If you have a previously established relationship with a vendor, by all means stop by their booth to say hi. I mean, we're people too and we like to see old friends and make new ones. But again, respect their time and space. If the booth is crowded, come back later. Oh, and when you do get a chance to visit with them (briefly), make the appropriate ooohing and aaahing noises that indicate their new stuff is wonderful.