I'm by no means an expert but I've got a few pointers:
- Try to have roughly the same number of people in the playtest that you'll have in your actual game. I know that this is a bit tricky because you won't know exactly how many participants you'll wind up with until closer to Game Day. I also recommend determining which of the PC's are "primary" and have abilities that you feel are essential and those which are "secondary" and are not absolutely required for the adventure to be fun and successful.
- Keep an eye out for any areas where the game drags at all. Due to time constraints and the general nature of the Game Day environment, you want to keep things moving at all times. Places to pay special attention to would be any unusual rules situations (like an enemy that uses an obscure spell or makes some sort of special attacks), NPC interactions and any place where the plot branches or requires the PC's to "find a clue" before moving on. Also, try not to have the ongoing adventure hinge on a singly event taking place. If the PC's only have to make a single, DC 10 Tracking roll to follow the baddies to their hideout, they either won't think of it or will roll a natural 1. Never fails.
- If I'm not sure how many players I'm going to have, I find it easier to scale a combat that includes a lot of mooks and mid-range bad guys rather than a single big bad guy. I can always leave out some of the mooks but it is tricker to nerf the big-bad if I wind up with fewer players than what I'd anticipated (and even if your game fills completely up it is possible that you'll have somebody be tardy or no-show). Same principal applies to ramping up the encounter if you have a couple walk-ins at the last minute.
- Styles will differ and this is not gospel, but I try to make sure that the encounters early in the adventure will not likely kill off a PC or that I have some kind of backup plan if they do take a casualty. It really stinks to have your character killed off 45 minutes into the session and be left with little to do for the remainder (though, thanks to our partnership with Trinocon, there are always board games and such going on). I try to design encounters early in the adventure that let the players "warm up" a bit to the abilities of the PC's you've given them, without requiring that they play flawlessly and use every ability optimally in order to be successful. Save the encounter likely to kill some of them off for the big climax near the end.
If there is a PC death, I try to have some sort of backup plan like an NPC they can take over, resurrection or something like that. In my last Orcz adventure, one of the PC's had a pouch of "Zombie Powder" that would bring a PC back to "unlife" with all their former abilities for a number of hours equal to their Con. After that they became a mindless normal zombie. This gave me a safety valve in case of a death but still had some consequenes.
- As Henry mentioned earlier, try to have a "throw away" encounter that you can leave in or take out depending on whether you're running ahead of or behind schedule. And, on the matter of the time frame, when I'm playtesting (or gaming in general) with my normal group of buddies, we tend to screw around and table talk, discuss video games, wives, children and football more than would ever happen at Game Day. Take that into consideration when you are judging how long the adventure will take to run.
- At the end of the session, ask for feedback from the players as to what they loved, didn't love, hated and how you could turn the "fun dial" up to eleven. This is going to be your best guage of what needs fixing and what needs to be left alone.
- As a general principle, when in doubt opt for fun and playability over balance and adherence to the rules. Remember that, unlike running a regular campaign, you're not setting any bad precidents that you'll have to live with later. You know for a FACT that any bad rules calls or Monty Haul treasure give aways won't haunt you past the next few hours. So be generous, outrageous and over the top. No treasure is too big. No explosion is too big.
- Make sure that you are having fun too. I find that this isn't hard to do with big explosions and treasure everywhere.
Hope that was of some help.
