For example, someone may have really bad hands or may bounce at the trot. Look for something good. Maybe their horse is moving well, or has perfect transitions. Maybe their tack is spotless because she was up all night polishing her stirrups or the silver on her saddle. Unless you have perfect rides all the time, at every show, keep quiet.
People learn by example, so be a good one.
Clap for a rider even if he or she had a difficult ride. Tell someone they did a good job. Absolutely do not sit off to the side and whisper about someone's ride. Because eventually it will happen to you, and it doesn't feel good. No one goes to a horse show to have a bad ride.