I've said it before, but be sure to bring a roll of toilet paper. You'll understand why later.
Having some green tea at hand will go a long way to preserving your voice.
I said first-time booth monkeys, not first-time con-goers. 
That's what I meant. It's super important to have a roll of toilet paper, and preferably a block of Parmesan (not necessary, but recommended), when working the >G booth. Just for the first day, though. You won't need it after that.
You also need that guy's artificial leg...
sigh Imprimis5… That's only when you are Sidekick for the con.
When it's me, they just need Sharpies. 
Hey, I was giving the easy list. When I was handling GenCon this year, that Parmesan was required.
And thank you, Spiff. There may be a tiny raccoon inside of me somewhere, cursing to be let out in several languages.
Have some kind of hand sanitiser nearby, and use it often.
Make sure you drink a lot of water, and don't forget to eat.
Get used to playing the same setup over and over, so find ways to ham it up and keep it fun! You don't want to be boring people who've never played just because this is the 50th time you've played this setup today.
On that count, don't forget to bring food and drink.
throat lozenges
Care to guess how many times I've played Legacy in First Chair, fighting Baron Blade on Insula Primalis?
GALVANIZE ALL THE HEROES
I'm surprised that Anna hasn't corrected you on this point yet.
Pydro to the rescue!
See, I don't have to do anything anymore. If I wait long enough, someone will post a link to one of my old comments.
Yeah, definitely be ready to be in a support role. In running a demo, our job is to help the players have a good time. We shouldn't be doing that by stealing the Big-Bad kill or otherwise taking their joy. 
That really makes Legacy the perfect choice.
I kept having people that wanted to play Legacy. at PAX East.
I played Visionary a few times, but mostly I just ran the game while 5 people played.
I am fairly certain you have to play with the Prime Wardens promos at PAX Prime.
Against Akash'Bhuta. And those demos better not last longer than 20 minutes.
heh... playing legacy in demos. Having Thokk in your hand and a flying smash and -never playing them-. Playing give you powers! give you damage! Take your damage! SUPPORT
its still fun to see new people get the sudden combos of players, or when a new player with Expat has 3 or 4 guns out and draws Unload, and their eyes go WIDE.