I've been shooting combat matches for years. Iron sights for all three guns, can't think of too many things I'd rather do on the first Saturday of each month!
1. Always carry extra ammo in your pockets. Last match, I finally loaded a short mag. It was short because I allowed myself to be distracted and failed to upload it and let it go back into my mag holder. Yes, I was astounded at myself and that cost me time. I WILL NOT do that again!
2. Unless you are super-competitive, don't try to beat the other guys, especially those who are always in the top three places. Shoot against yourself and how you did last time. You will improve and will likely catch those other guys.
3. Remember, you're out there to have fun and become a better shooter. If you shoot combat matches on a regular basis, you're probably better than most of the other shooters out there, even if you're not in the top half of the monthly crowd. At least you're out there and they're not!
4. Practice, practice, practice. If you can put some rounds down range the day before the match then do so!
5. Volunteer to design scenarios. Then, design them so you have to work on those things your really suck at. You can only blame yourself for how hard it is and you make yourself work hard at making yourself better. Many of the other shooters will "complain" about "Who designed this nightmare?" but will really be glad you did because they also suck at the same things you do! Weak hand, prone pistol, weak side shotgun, etc.
6. Have fun!
7. Take your time. A 10-second round with 5 "mikes" is worse than a 20-second round with no "mikes."