1) Never criticize your coaches if you aren't willing/able to volunteer to coach. The vast majority are doing the best that they can under the circumstances. My only exception is if the coach is doing things that are unethical, immoral, or that are endangering player safety. Unless there are children in imminent danger, you can address your legitimate concerns with the coach privately.

2) Never yell at or humiliate your kids during games or practices. IMHO, getting humiliated by your parents in front of your peers is absolutely destructive to a kid's emotional well-being. My only exception to this rule would be if my kid is so out of control that parental intervention is required. I haven't had to step in, but I've seen parents that had to.

Parents need to realize that they are there to support their kid, and not try to re-live the glory days through their kid.