1. Mechanics: get an angle, be in the right place to see the play. That doesn't guarantee you'll see it, but it improves the odds and is one less ground for complaint.
2. Coverage: if you're off ball and it shoots out past you, get help. Don't ball watch because you might miss an OOB.
3. Call: For NFHS, at least, use the stop clock mechanic. This gives your brain a moment to process what you've seen. Point and announce the color: if they don't match, again, you'll have a moment to process what's right. Slow down your calling and you'll make fewer mistakes.
4. Help: if a partner comes to you with info, take it, change the call, and move on. You should pre-game that nobody corrects an OOB unless they're 100% certain. Plus, now coach can go talk to your partner.
5. Coaches: they don't get a long conversation about OOB calls. "Coach, I had it off white's foot. Let's go." It's a judgment call, and they don't get to argue it. If they persist, I use a simple "You might have seen it differently, but that's enough, coach." (I might converse more about block/charge because somebody gets a foul toward their 5 for that one.)