They are definitely a mind-over-matter exercise. Some of the points that have really helped me:
1. Active shoulders, completely locked out arms. If you don’t have your shoulders in your ears and your arms bend at all, you’re going to lose it no matter what the weight.
2. Filling your belly with air (not your chest) and holding your breath during the down and up is key. Not only does it protect you from injury, but it automatically makes you more stable on the way down and up. Just don’t pass out.
3. Ass goes back, not down! This is the big one for me. If your OH squat starts with your butt going down instead of out, you’ll likely fall on it and have to dump the weight.
Those are the three major things that I try to focus on when OH squatting. Hope that helps.