I have been working GenCon for AEG since 2000, missing two years along the way, mostly demoing games, and most recently back to Games Mastering. I have run per-packaged modules before but for the last three years, I have been creating and GMing the adventures for GenCon. So, this advice is primarily for GMs.
Be Prepared, have what you need (books, dice, notes) ready, but also have the minimum the players need as well: character sheets, dice, pencils, paper for name tents, and whatever else they may need for the adventure at hand.
For example: Legend of the Five Rings (L5R), which I GM, requires a fair number of D10s, so I have a bag of 30 for my own and the players’ use. (Though I do usually manage to forget something, this year it was extra pencils.)
Let the Players Make Informed Choices, player do not know your style of play and (in some cases) the setting and rules, so do not be afraid to stop the game to explain the situation, risks and rewards to the players. Start with making sure that the players are able to make an informed choice as to the characters they are playing, for something like Pathfinder Society where they build their own, this is not so much of a problem but when providing pre-generated characters it can be more of a challenge.
For example: Here are the cover sheets to the characters I used for my introductory L5R game, note especially the “Play this character if” section, that all of the advantages, disadvantages and school techniques are defined (so the player does not need to look at the rulebook), and, lastly, there are brief roleplaying hooks for the other characters. The “Play this character if section” and defining the Ads/DisAds was new for this year and very well received so I highly recommend a similar build for other GMs as it allows the players to find a character they want to play easily and then have to tools to do so immediately.
Let the Players Set the Tone, now, my default mode for games is drama to action movie, trying to move the plot along and playing a fairly serious game. But at a convention, you need to adapt to what the players are interested in, I start with my default setting and adjust from there. If people want something light, play up the absurdities of the situation and let fun coincidences happens. If they want something dark, the brutal combat system of L5R -for example- will help provide that. Roll with the mood of the table and the game will be better for it.
Be sure that you have eaten before hand and have drinks to hand, being hungry will throw you off your game, so eat, and not being hydrated will be rough on your body and your voice (which you will be using, a lot, as a GM at GenCon, as it gets noisy), so have water or other drinkable to hand. Throat drops can also be helpful.
Well, those are the big things that I have learned and that I think have made my games pretty successful over the years. I hope they prove helpful to you as well.