What actually goes into that 5 hour, 100+ person "one shot"? There are 19 other GMs, some who won regional qualifiers as I did, others who have already earned the title Iron GM (at least competitor has done so twice), still other so emboldened by their skill (and ego) that they bought entry into the contest; hat will it take to trump their efforts and claim victory?
Perhaps starting with what type of experience will earn that vote? The story MUST be fun for the players- there is no two ways about it. But it also must be fun for the GMs, and in that balance lies the first pitfall: find your own source(s) of fun in running games and find ways to share that joy with the players! If you love colorful combat descriptions, then use them every time and involve your players. After one or two rounds of combat, after a particularly successful roll, pause for a moment and see if the player picks up on the cue; if not start the description but focus on the player and lead them to take the reins.
This is a two-way street, however; quickly observe what the players enjoy and run with that - infuse massive energy into those game elements. That way, both you and your players are getting copious amounts of what they love in a game. A great way to tell what's important about a character to the player is to ask for a 1 or 2 sentence description. Race, class, and gender will obviously come out, but there may be nuggets of what make that character tick. Seize those and incorporate them into your adventure.
Second, the story must start quickly, move forward, and END (preferably with a bang). I started my last session in combat - the players were still noting details about their characters while we started walking through an oppressively humid rainstorm in a jungle. I described a quick outburst of activity and asked the fighter was he was doing - no initiative, no PC placement, no map. Just start - BANG! Grab player's interests early and hold on for dear life.
Don't be so in love with your own story that you won't cut it to get to the end; I don't care how compelling the plotline, an unfinished chronicle faces nearly insurmountable odds in claiming victory. If anything seems to be in the way of completing the story, kill it like a plague-baring mosquito. Any encounter running long should be shorted or ended as quickly as possible; players won't know the exact number of hitpoints or feel cheated that they didn't do all 104 points of damage to your rotting creeper; they WILL resent not playing out the last battle or revealing the identity of the traitor to the king.
The last encounter should be monumental - incorporating fantastic and/or exotic terrain, large numbers of foes to deal with in one way or another, some emotional payoff, and a REAL threat of failure. If you leave any trick in your arsenal out of this fight, you've missed a golden opportunity. Make the players sweat, give them real decisions with repercussions for mistakes or bad luck, and bring the heat.
By this time two weeks hence, I'll be writing about my own Iron GM experience.v And I'm confident that my players will remember it for some time with fondness.
That means I win.
