Just some thoughts I wanted to share for when you are holding the timer:

1. Learn the damn rules and keep the rule book nearby. You will eventually interact with a Range Lawyer (defined as a shooter that will try to eek an advantage by arguing the rules rather than shooting better) and your command of the ins and outs of IDPA will come in handy to shut them up and keep the match flowing.
2. Be polite, but when safety is on the line be a polite asshole. You are a Safety Officer first and foremost. The well being of fellow shooters is your first and only consideration. Do not be afraid to use a command voice and control any unsafe shooter. If they get mad, it is just a feeling and will eventually get over it.
3. 90% of NDs will happen during the “Load and Make Ready” and “Unload and Show Clear.” Pay special attention to finger position & muzzle sweeps while loading. Visually confirm that there is no magazine in the mag well and there is no round in the chamber when unloading. If the shooter is one of these guys that do not wait for your commands and does the whole unload thing plus holster, command them to unholster and repeat the whole thing till you are satisfied. They give you any crap, ding them with a procedural for unsafe conduct and send them to the Safe Table to have a chat with the Safety or Match Director.
4. If you are mentally scoring the competitor while he shoots, you are not paying attention to the gun. Your eyes must be “glued” to the shooter’s gun and arm. Leave scoring, cover violations and the such to the Score Keeper and concentrate on safety. If you catch a violation without being distracted from your main objective, call it but do not seek them.
5. Don’t be afraid to give FTDRs or DQs when required or needed. We all hate giving a DQ for a shooter’s mental fart or weapon malfunction but we are required to so. Most shooters will understand and will not protest, some will so stand firm. But do not be afraid to use them as behavior modification tools against those who want to “game” you or disrupt the match. I had a shooter who would not stop complaining about a procedural I gave her for not having proper equipment and I told her to take it to the Match Director and proceed to set up with the next shooter. While the next shooter was loading his gun, the complainer got in between us to keep arguing her case. I gave her a DQ for unsafe conduct right on the spot and ordered her uprange. After the customary cussing, she packed, left and never came back. Our club is much better with her gone in my opinion.
6. Never let a New Shooter go first & be gentle. By now you know they are nervous as hell. Let them see old shooters take on the stage, do the extra effort on explaining the stage and then let them have a go at it. When the New Shooter is done, take an extra minute to point out any deficiencies and congratulate him for joining the fun.