Discipline?

Everyone has an opinion on discipline. But who is right? In the end, I think we all have an inner instinct that we should listen to. Whether it is your children, your animals, babysitting others' kids, or walking into Wal-Mart, there is a line for the acceptable and unacceptable.
How do you know when the line is crossed by someone else? When is it ok to speak up to someone? And furthermore, are you confident that you are right? As a mother of a 2 year old, I struggle with discipline daily, of course. And I have never had anyone say something to me about being in the wrong, so it is not at all like that kind of thought in my head. But I wonder what methods people have tried, have failed at, and have worked? Lets face it, we can list on 5 fingers parents we have seen or know that try and fail at discipline. And then there are the parents that are admired for how "together" they are. I have truly lost sleep over what they are doing... So let's have some share time, a show-and-tell for parenting if you will. Failures, Methods, and Successes are welcome here!