I think a lot of people have hit it already, but having a positive attitude is really important. The other thing for me was positional shooting UNDER TIME. For some reason, I was really concerned with the basic marksmanship of the match - in other words, being accurate enough. I was surprised to find my accuracy was not a problem at all. There wasn't a shot I couldn't make all day long. I had good dope and practiced positions. What I failed to do is practice any of that under a clock. When the rubber hit the road, I failed to even take about 40% of the shots because I couldn't get into position and acquire a sight picture fast enough to even squeeze the trigger. It was laughable at points (that's where the good mental attitude came in!). It dawned on me that I would have been much, much better off if I spent a few hours dry firing on the clock with a timer. Getting into position, getting a site picture, squeezing the trigger, move positions, repeat, etc. That would have been the single biggest improvement I could have made.
My first match was late last year, and with all the covid stuff I haven't been able to shoot one since, but I am practicing changing positions, albeit in the house until the range opens back up.