Congratulations on making it to your first Pro Tour as a judge. You're in for an exciting time and will get to meet a lot of great fellow judges from around the world. You'll have a lot on your mind, but here are five things you might not have thought of. No judging philosophy here, these are the practical aspects of being a judge.
The Judge Shirt
You'll be wearing the black-and-white striped DCI Judge shirts and will be given an opportunity to borrow or purchase one at the event. However, a Pro Tour is a multi day event, and you'll be wearing that shirt for three days. You could invest in a whole bunch of judge's shirts, or find an overnight laundry service, but far easier is to bring a pair of undershirts. Wearing an undershirt each of the first two days, you'll still have a reasonably fresh shirt to wear on the final day. Of course, the undershirt should be black or white - my personal opinion is that a black one looks better.
On Your Feet
It won't occur to you when you first walk into the venue, but the floor you're working on is concrete, occasionally covered by a very thin layer of industrial carpeting. Eight hours of standing later, it'll be the only thing you can think about. The impact on your heels of walking on that floor for all that time can get very painful. Not only do your heels get sore, but the shock impact will eventually lead to hip and lower back pain well as fatigue over the course of a multi-day event.
Gel insoles, even the cheap ones you can find at your local grocery store, won't solve the whole problem, but they will help ease the impact of walking on the floor. They provide an additional cushioning layer between you and the concrete and absorb some of the vibrations that would otherwise damage cause pain and joint trauma.
It is very important to stay hydrated during a major tournament, but unless you are consciously trying to avoid it you run a risk of dehydration, especially in a warm venue. The problem is that by the time you realize you're dehydrated it's too late to address the symptoms, and re-hydrating takes some time. Mild dehydration can cause light-headedness and a loss of focus, not what a judge needs when they are called over to adjudicate a difficult ruling.
A preemptive approach is best. A good rule is to try to have one cup of water between every round. Wizards helpfully provides water coolers at the major events - don't think that they're only there for the players. Even if you don't think you need the drink, take it. It will you in the long run. And yes, you should take bathroom breaks.
All result slips at the Pro Tour are signed by a judge. The players will call you over to a match and hand you the slip. You will verify the final result with the players, then sign it and bring it up to the result slips box. The key thing to remember is that the scorekeeper will not recognize your signature. Print your name legibly on the slip so that if there is a problem they know who to consult. This is harder than it sounds - your natural instinct will be to scrawl your signature on the line.
You Said What?
While judges strive to make the correct rulings at all times, it does not always happen. Errors happen at even the highest-level events and every judge there will admit that they've made them. Once you realize that you've made an error, the most important thing to do (after fixing it) is to let it go.
This can't be emphasized enough. A judge makes a bad ruling and lets it hang over them. It affects their focus and confidence in making future rulings. The effect is magnified at a Professional event because of the stakes involved and the presence of so many experienced judges. The good news is that the rest of the judges aren't out to get you and won't hold it against you. They've all had it happen to them as well. If you're still kicking yourself an hour later, you're not doing your current job as well as you could.
As I said, these are practical tips. Your fellow judges and the head judge will have lots of other suggestions and advice on how things should be done. Stay alert, listen carefully and have fun.