I was in Izumi's artist alley in 2007, 2008 and 2009...and I'll be in again for 2010. So here are a few things I've picked up that might help a bit. (Get ready for some tl;dr!)
Having a range of items is definitely good. Prints are what most 2d artists are going to have, but the keychains, buttons and mini-prints are a great idea. Things you can sell for a couple of bucks a piece. Sometimes people don't want to drop a lot of money at one time, but they won't mind parting with $3 for a small print or a couple of buttons. After a while, this tends to add up in your favor.
As Celia said, offering commissions is also a great way to make money. I never fail to pull some cash that way. Just make sure not to bite off more than you can chew, as it's a good idea to get everything done and to your customers with payment received by the time the convention is over. Some people might be alright with paying you in advance and having you mail a finished piece to them after the fact. Just make sure you have their address along with an email address or phone number for contact purposes.
Know how much you want to charge in advance for your commissions, as well. I usually start at $10 for a single character pencil sketch and work my way up from there. Been times I've pulled in $40-50 for colored inks finished in Copic Markers and prisma pencils. Typically I'll tip the scale up or down $5 one or another on my set scale depending on what's being asked for in terms of complexity. Mechanical stuff takes longer than human characters so I usually charge just a bit more for them, since they eat up more convention time and reduce the number of overall commissions I can handle.
Keep track of how much you make using a receipt book or notebook. I usually write down what I sold and for how much, that way I can keep track of what's popular and I have the info on hand to compare to my earnings when I count them up later. This also helps minimize confusion for tax reporting purposes.
And speaking of keeping track of money... Change for your cash box (or whatever it is you're keeping your funds in). It's always a good idea to have some $1's, $5's and a few $10's on hand so you've got change to give someone when they pay with a larger bill. It's not a bad idea to sell things in whole dollar amounts so you can avoid dealing with coins, as they're heavier and harder to keep track of than bills. I've seen other artists deal in coins, but to me it's a hassle and something to avoid if possible.
One problem I always ran into early on was how to make sure the art I sold my customers didn't get banged up. Sometimes people will have folders with them to put art in, but often times they won't. The best solution I've found so far is cheap page protectors. Get a package of 50 at a local Walmart for a couple of bucks and when someone buys a print or you finish a commission, slip the goods in there so that they don't get dinged up.
Business cards! These are great to have on hand if you either order them off a site online or make them yourself. Just something simple like your name, email address and website. Someone might not commission you or buy your art at the convention, but if they go home and know how to locate you online later, they'll have a chance to see more of your art and possibly commission you at a later time. Plus having business cards makes it easy to give out your contact info to your fellow AAers.

I've picked up several new people on my DA watch list thanks to easy info exchanges at conventions via business cards.
Bottled water. Or juice or whatever your preference is in terms of beverages. I've found it's really easy for me to talk myself half hoarse at an AA table, what with all the conversations that get going about art, anime, movies and everything else. Keep something around to drink.
And finally...
WASH YOUR HANDS. Or keep some Germ-X or something similar around your table. Trust me, this is nothing against anyone who exhibits in AA or visits or anything. But we all know that Con Plague gets around and it's really easy to shake a bunch of people's hands, handle their money (and oh god, money can be so filthy, you know this if you've ever worked regular retail as you never know who had the bills before you or your customer), use your art supplies, open doors into panel rooms that like two hundred other people have touched, etc, and then forget to wash your hands before rubbing your eye or eating something. Which is very easy to do when you work an AA table and you've got commissions to do and customers to talk to. I picked up Con Plague in Tokyo in Tulsa's artist alley in 2009 and it took more than two weeks to get rid of. I love conventions, I love working AA and I love drawing for people and meeting them, and no one means for anyone else to get sick, but it does happen. And nothing is worse than getting the Con Plague before a convention is over and having to finish art while sniffling and dealing with that itchy throat feeling. Washing your hands with some degree of frequency or using hand sanitizer can help cut down on your chances of getting sick. I also start taking vitamins a few days before any convention now just to boost my immune system. My personal favorite is stuff called Emergen-C. Most grocery stores carry it and it's a powder you dissolve in 8 oz. of water and drink. 1000% vitamin C. I swear by that stuff.