What pops to mind for me.
First of all know what you want to get out of the tournament. I assume you’re looking to win it, but some people are just looking to hang out with good friends and play a fun games, and some people are somewhere in the middle. If you’re in it to win it, then remember that and act accordingly and maybe don’t take a shot everytime you lose a psi game or whatever fun stuff is going on.
Know your body. Know what foods you need to eat to keep your energy up, and how much. Know how much sleep you need to get the night before. Know what to do keep the adrenaline going between games, because if that wears off it’ll knock you out.
Play a lot, and practice playing for long stretches. Other GNK tournaments or stores are good for this, go to as many of these as possible. Or OCTGN/Jnet binges on Sat/Sun (or whenever you have a free day).
Know your decks and the matchups thoroughly. If you’re unaware of what your opponent’s deck is doing and what yours can do to counter it 1) You’re going to be burning a lot of mental energy solving problems that you should already know the answer to 2) You’re going to be responding/thinking in broad strokes rather than smaller details. As in if you know what breakers are most important against this type of corp deck, you’ll be thinking about when to get them, rather than what to get.
Automate certain processes and plays. I find it so much easier to track my and my opponent’s clicks using trackers, and it saves mental space (and very often I catch other players forgetting clicks, even good players). Any little mnemonics or actions you can take to keep your thought processes on the board and not taking up space in your head. Have certain lines of plays with certain cards. Like generally with Jackson I want to install, draw 2, draw 2, discard to an ideal hand. Now when I draw Jackson in a game I ask not what should I do with this Jackson, but whether there’s a better line of play than the ideal. I find the second question more specific and easier to answer. Also planning. Take a few seconds at the start of each turn to a) check any start of turn triggers, and b) plan what you’re going to do. Even though the plan might change (drawing an important card, running into an ambush) it’s easier to reformulate a plan than constantly playing hand to mouth.
Also play the game and then move on. If you lose to a stupid mistake don’t dwell on it. This can be difficult to do.