The most important rule I can give to anyone: Never EVER do a practice or training hold the same day that you plan to compete. Don't do any shoulder or chest exercises either. You're using a really small muscle group and even if you feel fine, it takes a while to fully recover. Since this is an endurance competition, you can't afford to waste any energy because you just won't recover fast enough, even after a few hours. To risk doing two holds in the same day just isn't worth it. You won't gain any advantage by practicing a little bit more and you definitely will hurt your time for the competition. I've seen it happen, trust me. Unless you are competing against me, promise me you won't practice the same day you have a competition. I mean it. Say it out loud right now.
That being said, keep your shoulder loose before you compete and make sure to keep your arms and shoulders warm, especially if you are competing outside. For most forms of athletic competition, you would want to warm up beforehand, but I don't recommend it in this case. It's too easy to waste energy and risk hurting your competition time, so don't do anything to exert yourself. Go for a sweatshirt and wear layers instead.
The night before a competition, make sure you get a good night's sleep and eat a LOT the day before and the morning/day of your competition. You don't want to go into it full and uncomfortable, but I can tell you for sure that "carb-loading," as ridiculous as it sounds, has made a huge difference for me and my performance. Remember, this is an endurance sport, so the more you can do to promote recovery and keep yourself energized during the competition, the better off you will be. My go-to pre-training snack is a ton of Greek yogurt with honey and fruit, but something like an energy bar or protein bar about a half hour before the competition is also a decent choice because it's portable and convenient and again, it's going to give you simple carbs and amino acids that will keep your muscles supplied while you are competing.
If you are in the unfortunate position where the competition you are entering has two rounds on the same day, well, it's going to suck. It may feel tempting to assert your dominance immediately and shoot for first place in your first heat, but don't hold for any longer than you absolutely have to during the first round. If for example, the top 4 times move on to the next round, absolutely do not try to take first place in the prelim round. Stop once you take the number 4 spot and laugh at the other a-holes that keep going while you save your energy for the finals. I promise you that every second you continue holding is going to shorten your time for the next round as well as making the next hold feel harder from the start.
Stay hydrated in between rounds especially if you sweat a lot like me and eat something with carbs and protein within 15 minutes of completing the first round. Energy bars and protein bars are a good option since they will give you easily digestible sugars and amino acids that will get into your bloodstream fast and help your recovery.