I would like to note that most of us do not confront or take confrontation well.
Some people are combative and it takes very litle to push a button so to speak.
Other people will let so much slide that when they finally do let it out, its not very pretty.
I don't have much advice for combative people, exept please stay away.
But for the people that should be a little more assertive try doing it in small things. Ask for extras at restaraunts. That small thing you let go because it doesn't make you angry? Don't let it go, but don't get angry.
You can simulate anger, but don't have to feel it.
When you master doing it, you wll find it very easy to assert your foot into someone's ass . . . to the appropriate depth.