This ^^. Learned that as a marathoner - don't eat anything, drink anything, or wear anything you haven't used in training. No new thumb rests, springs, mag holders, or other gear. Any modifications to your gear should be done enough ahead of time that you've tested them.
And all the other stuff too. Especially getting there ahead of time to walk the stages. You may get to see staff shoot as well and see some approaches to the stages. Diagrams can be misleading, and sometimes the stages get changed.
And shoot like you know how to shoot. You're more likely to have a good time and less likely to make mistakes if you don't act like you have to shoot like the cool kids.
One more thing - drop your mag before you rack the slide on unload and show clear. That error got me to Dairy Queen early on in my first major.