Preparation
The obvious stuff like getting your game prepared, needs to happen. Studying does help you. At the very least, it will help your notation as you are more comfortable thinking in notation.
It will also be a review of what you should be thinking about in the context of your game. The tactics, the center, use of initiative and the like are things that can be seen in the context of preparation.
Your health. This can be underrated. In the New York Open, I had untreated high blood pressure. I got tired toward the end of the rounds. I lost a game where I had an edge in part because I was tired. I later got my blood pressure under control and my stamina got better.
Your head. Clearing your head of distractions is crucial. This important before and during the tournament. I remember a couple strong plsyers got distracted by Antonetta Stefanova at a tournament. She was pretty attractive and both players got distracted and they lost to her. Clearing your head of prior results, mistakes or moves is also important.
What to expect
1) Expect a fair amount of noise at the beginning of the round and a lot of noise at the end of the round. It never fails. Complete silence will not happen. It is not so bad at the beginning of the round, mostly from setting up the set and pleasantries. The noise at the end of the round is a bit unforgiveable. Most of the noise makers have already finished their games, so they really should keep quiet.
2) Expect some good sportsmanship. For example most players will let you see their scoresheets, even though they are not required to do so.
3) Expect to be pressed over the board, the bigger the tournament the more this is true. Your opponent is going to try hard to win, just like you. Some (if not much) of your preparation is not going to help you, save your confidence in your game. It is often too unpredictable to know how things are going to come out. Spending as much energy as you can at the board is important. At the Fifth Corps Championship and the US Army Europe Championship, I got good results because I worked hard at the board. Similarly, I had too much confidence in my preparation and I lost at least a point in the 1996 World Open and a half point in the 1997 World Open that would have at least had me money to the extent of getting my entry fee back and about $50-$100.
4) Expect to enjoy the atmosphere. Again the bigger the tornament, the more this is true. My first and one of my biggest impressions of the World Open was that I saw players playing blitz in the lobby. This is not common. But it is common at big tournaments. I enjoyed the atmosphere at all the big tournaments and most of the small tournaments. I really liked the National Open that I went to some years ago. It was very pleasant and I will one day play there again.
5) In big prize tournaments, expect sandbaggers. They might be from another country, but are a bit more likely coming from the US.
6) Although it will not always happen, it often does happen that the souvenir chess boards at the big tournaments (National and US Opens) ususally sell out. If you do not buy them in advance, you can expect to get nothing.
This might be obvious, but when you play at tournaments your opponents and your TDs expect you to know the rules. You are also expected to bring your own equipment.
How to behave
Be polite. The larger the tournament, the more important this will be to you.
On a practical level, it is very useful to be on good terms with your former opponent. At the very least, you want them to do well. They cannot play you again and they can pressure the rest of the field. If there is a shot at a trophy or a title, their performance can be important in any tie breaks. They can also give you some sense what your next oppenent might play. They might have played them in a previous tournament and/or might have sat near them in a previous round.
Conversely annoying your opponents (and their friends) can be a very bad idea. There is a bit of an urban legend from the World Open. Some years ago, an obnoxious player was playing in one of the sections. In the penaltimate round, the obnoxious player's opponent left the tournament hall to use the restroom. When he came back, the other players (falsely) asserted the obnoxious player had touched one of his pieces. As the other player had a pawn that was attacking one of the obnoxious player's minor pieces, this was decisive. The TD backed up the claim and forced the obnoxious player to move another piece. The obnoxious player lost.
This list is hardly exhautive or complete. But I felt it is a good topic for comment.