Definitely volunteer. First, have fun! Being flexible to help out is always good, but also realize that demos are the area of biggest need. Enjoy the game enough that you can do Baron Blade repeatedly, because he really is a great villain to show off many of the mechanics of the game while being easy enough for most newcomers to beat him. Grab people who look interested, because they probably are. Mostly, explain thoroughly and be patient, because the game will sell itself if you set it up well.
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Volunteer.
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Wear a tuxedo.
And yes, Rabit, her name was Ariel, you remembered correctly. 
I'll try and start uploading all of the cosplay photos I have soon.
First and formost, make sure you enjoy the game. Otherwise it’s going to be a long weekend. Be paitent and delibrate with your demos, and be ready to feild a lot of questions. Don’t be afraid to offer new players advice, but don’t play their turns for them. I actually had the new players play with their hands up and the first turn I would offer a bit of advice, but after that I stayed out of their turn unless they asked. And lastly, be enthusiastic. That energy will quickly transfer to the players who will soon be enjoying the game.
I just joined the forums so I could post this.
Prior to GenCon I had known people who really liked Sentinels, but I had yet to play it myself. I got a demo and long story short I now own everything that can currently be bought for the game, and all the promos they brought to GenCon. All of my friends and I really like the game a lot, which is quite an achievement, so kudos to everyone who had a hand in making it what it is today.
What I wanted to say was how cool it was to see LITERALLY EVERYONE in and around the booth be so excited to show new people the game, and so eager to share what they love about it. I've had demos where the people giving them didn't seem to care much, and that leaves a negative impression on a game for me. So thanks to everyone who worked, volunteered, and played at GenCon, you made several new fans, and I'm sure we aren't the only ones 
Welcome, GoodGuyJim - glad to have you join us!
Welcome to the community, Jim!
For those interested, the Geek Dad had a great set of Gen Con photos that included some from the >G booth. And the note on the last one is absolutely correct.
The booth was awesome. The tables were always full and Christopher and Paul seemed to be full of energy, even on Sunday. I spent most of my Sentinels Points to get a couple of the promos for a friend of mine who just got into the game. I wish school didn't start so early sometimes. My school didn't start until August 14, but it's tough for me to take more than one day off that early in to do the GenCon thing. Otherwise I'd have loved to have volunteered for a few days.
I had a hoot demoing a bajillion games over the four days of GenCon. I can't even recall the huge numbers of new players that immediately popped over to the register to purchase after playing. So many people were just completely awesome to watch play, discovering different things about their character as they went. Gah! What a joy!
I do, of course, feel a little bad for my favourite Vengeful Scientist, as on more than one occasion, he barely got a single flipped turn before being decimated by the heroes. L'sigh. I am proud of him, though, standing against hundreds of games and almost pulling out some wins with a few shock turns thrown in.
And kudos to the booth space. It was an impressive set, and not a single person could say they didn't notice Greater Than Games at the convention.