My son is 20. My daughter is 17. Both are polite, courteous, and respectful of adults.
I've lost track of the number of times other parents have pointed out how well behaved they are.
The last time I spanked my son, I think he was about three or four, and I realized that I was doing it out of frustration more than anything else. I never did it again after that, and I've never laid a hand on my daughter.
Discipline is more than simply punishment. Beating your kids isn't some panacea that will magically make your child into a responsible adult.
Of all things... Consistency is the key. That and never EVER EVER compromise on a decision. Once you let them think they can negotiate with you, you're done.