The "Matheny Manifesto" is a good example of a well thought out parent meeting guide. Obviously, he has been "around the block" and more than qualified to lay something like this out to present expectations for his particular group at that particular time. It is important for a coach to have such a thing and there is plenty in his version that can be (and is) used as a starting reference point.
That said, there are no perfect coaches and there are no perfect sets of parents. We should all be more realistic with this whole thing. It should not be required that the coach is "beyond reproach" as some have suggested... set a perfect example in order for the parents to agree to adhere to whatever guidelines are put in place. Hell, most are volunteers, either completely or to a large extent. And, like others, the one thing I don't agree with in Matheny's rules is for parents to sit and be quiet. There is supposed to be cheering at games. Silence is really boring for everyone. Yes, there should be helpful guidelines provided at the parent meeting so that those parents so inclined can be less likely to be "that parent". And, most often, there needs to be specific rules (stay out of the dugout, no verbal coaching your kid when he is hitting, no negative stuff toward other team and umps, etc.). But, beyond that, let people enjoy being at their kid's games and be themselves. Hell, that's half the entertainment.
Most people will naturally cheer for their own team and their own kids, whether positive results are due to positive actions by that team or mistakes by the other team. No big deal. Yes, of course, taunting a little kid after a mistake is taking it too far.
If you ask a kid if he wants his parents cheering, what do you think he is gonna say? He's a kid (or teen). Reality is that when it's all over, he'll be glad someone was in the stands cheering for him. It may be years later but...