You've earned some level of respect in your local community. You've learned to work the TOs in your area. You think you know the rules relatively well. And now you've caught the eye of the DCI.
You've applied for sponsorship to your first Pro Tour event, and have been accepted. I'm going to tell you what you're going to need to know to make the most of the experience. It breaks down to three simple things: Know the Game, Know the Environment, Know Yourself.
Know the Game
This may seem oversimplification, but make sure you know the rules. As good as you are with the rules, rest assured that your knowledge will be put to the test during the weekend. Study not only the rule book, but use your online resources as well. The DCI Judge page is an excellent place to begin, but there are other notable sites as well. Drop into mIRC chat rooms like #mtg, #mtgpro, and #mtgwacky, and listen to discussion about the game from its highest-level players and Judges.
Study also the Penalty Guidelines. You've never before been at an event under REL 4, and things are a bit more stringent.
The Head Judge will explain specific policies and procedures that the floor judges are to follow
There are some Judges who don't play much. Don't be one of them. Play as often as you can. You'll be surprised as how much a little play acuity can assist your judging when it comes to assessing a tricky situation.
Know the Environment
This is actually broken down into two parts: the environment of the Pro Tour, and the format of the event.
The Pro Tour is like nothing you've experienced. Everything is at an order of magnitude higher than you're used to. Everyone-player and Judge alike-was at one time a "big gun" in their local community. You're truly among the best of the best in both regards. Egos are larger (and more fragile), tensions are greater, and the stakes are far higher. Things are going to be far more serious than your local events. Be sure you're ready to rise to the occasion.
A major difference between the Pro Tour and your local events is your purpose. Locally, part of your job is to teach the game while you're refereeing it. At The Show, your role becomes strictly disciplinarian and enforcer. Your primary job is to help provide an environment of stability and fair play for everyone.
When it comes to knowing the format, keep abreast of the latest developments. Get to know the metagame. There is boundless discussion of what decks are being played, or which are being developed. Look at all the cards from those most popular decks and see if there are interactions that might cause some trouble-not just for you, with your decent rules knowledge, but for players who might not have your depth of insight.
If the event is Limited, know which cards are most popular so you can look for difficulty trends, but don't dismiss cards that are seemingly unpopular. Pros, always searching for an edge, are likely to try just about anything.
Finally, especially if it's a Limited format which you've never run (Rochester Draft being the one that gives people the most trouble), learn the mechanics before you show up on site.
Know what you know and know what you don't know. It's okay that you don't have guru-level knowledge of the rules. Figure out, perhaps with the help of your local Level 3 (if you're lucky enough to have one), where your weaknesses are, and seek to improve them. Whether it's the interaction of continuous effects or how to effectively communicate with players, knowing your weakness is the first step to overcoming it.
Traps to Avoid
t's natural to want to distinguish yourself at the game's highest levels, but the single best piece of advice that I can offer is don't try too hard. Let your abilities speak for themselves. It's smart to want to hang around with and learn from the Judging communities luminaries, but resist being a fan and concentrate on being a tournament official.
That goes for both other Judges and the players. Many first-time PT Judges fall into the trap of wanting to watch the matches of the Tour's top players, or the matches at the top tables. Resist the urge and give every player your full attention.
Your first Pro Tour will be a great learning experience. Go into it with your eyes wide open. Know the game, know the environment, and know yourself, you'll make the best of it.