Best suggestions I have:
1) Find something that will appeal to as many people as possible to run. What I'm about to say won't help now, but it might help in future Gamedays: When you are at the Gameday, ask people what they might like to see run. While it's cool to ask online, the people who respond aren't always the people who will finally show up when all is said and done. In absence of empirical evidence, pick a perennial favorite game system, or go the other tack and pick something totally off the wall but with cool subject matter, something people will say, "I've always wanted to try that!" Including a door prize, while not necessary, can also help draw people.
2) When Planning the game, try to come up with an interesting situation or set-piece that serves as the climax. A fight in a dungeon is one thing, a fight in an apartment complex with 2 score unkillable zombies, as tenants run around fighting or screaming is another.
Write up a couple of elements that can be included or excluded as time demands. You want your 5-hour game to last at least 4 hours, just so the party who unexpectedly solves your game in record time aren't stuck with nothing to do for 2 hours. Conversely, the party that flounders and stumbles may need to pick up the pace a little after they get back on their feet.
3) If it's a game system you've rarely or never run, get your regular game group to playtest it for you. Playtest anyway, even if it is something you've run, unless you've run this adventure before. Identify the rough spots based on your playtest.
4) Ham it up like a mother. Anything from voice characterizations, to props, to colorful descriptions, make even games with boring rules systems fun.
5) One thing I frequently do is to create the PC's beforehand, so as to make them fit the scenario, highlight new game rules that are cool and give them exposure, and to ensure that those who are trying the game system for the first time have to do as little work as possible in playing. Spell out each and every special ability of each PC, and don't give them too many.
6) Finally, have fun with it, because people remember you by your game style as much as they do your personality and appearance.
Most of these are conclusions I've come to, as well as tips I've picked up from Rel, Piratecat, and other DM's I've met, and for me they've worked pretty well so far.