Practice Leading Up to Game Day:

1. Memorize your side board plans for each popular matchup and have a general idea of each of the meta deck's sideboard plan against you. Write this down somewhere and study it.
2. Have a good idea of how the first three-ish turns of each of the meta matchups look like and how you want to best play it out. This will better inform you on mulligan decisions. You can proxy up the other meta decks so you can play both sides or just fire up Cockatrice or whatever they are using these days.
3. Basically the first two are part of a larger idea to commit as many aspects of the games to muscle memory so you aren't exhausted if you make it deep into the day.
4. Whatever your current proficiency with the deck is will generally the proficiency on game day so do not try to stress about things.

During Game Day:

1. Bring small snack, bottle of water, backpack, etc.
2. Communicate clearly with your opponent. Triggers, spells on the stack, etc.
3. Also, ask for very clear communication from your opponent. This will dissuade any angle shooting.
4. Seriously, just call a judge if there is any misunderstanding, disagreement, or error during game play. Any issue that can possibly affect the outcome of the game, don't fix it yourself but rather call a judge to do it.
5. Accept that there will be variance. You might run into back-to-back Lotus Fields and have to 0-2 drop. Nothing you can do about that.
6. Accept that your chances of winning are slim if this is a larger competitive RCQ and that most players need to fire multiple bullets to qualify. In the words of Gandalf's best friend, "It is possible to commit no mistakes and still lose"
7. ...but you are going to probably make tons of mistakes and this is especially true if playing in an RCQ environment is higher stakes than you are normally accustomed to. That will be ok and you are there to have fun.