So many thoughts come to mind. A few important points I always followed (I have one 19-year-old daughter in college and one 17-year old son headed to college next year):
I try not to say "no" unless I have to, and then I don't reverse my decision (unless they are older and I've had a reasonable conversation...just like I would with an adult). When I say "no" my kids know it's important, and that I don't just say "no" without thinking about it.
I try to treat children with the respect I want them to show me.
Love them too much to argue with them...state the rule or decision and then tell them just that.
Always preserve the relationship.
I try to remember that discipline and punishment are two different things. Discipline is a great thing. Hitting, of any type, is not--in my opinion.
Also, I try to let "natural consequences" occur instead of punishment.
And Love, Love, Love them, unconditionally and with enthusiasm!