Well, as Ellana mentioned, discipline is not always physical.
I had been quite a little horror when I was a child, and let me tell you that the few slaps I got from my parents taught me more manners and self-discipline than all kinds of "you are not supposed to do this, honey" speeches. Because seriously, parents speak, child looks at them and all the kid is thinking is "blablabla... you finished yet?"
I'm not saying you are supposed to beat your child in a pulp. If the kid needs to be punished, the punishment must come swiftly (because punishing the kid for something they did week ago is, well, ilogical and useless) and must be also comparative to what did the kid do wrong. The kid brought an F from school? Alright, ban them from computer and revise with them. No need to slap them senselessly for that, as the only thing it will bring is scared child and more Fs on the way. They did something what they were nort supposed to do? Alright, immediately tell them they are going to be punished and explain clearly what they did and why it was wrong.
And seriously, do not punish the kid for something you yourself are doing - double standards never work properly, as far as I know, and it will only undermine your authority.
By the way, I'm not a parent myself, but I used to volunteer at chilndren summer camps as an instructor, so I know some things about "parenting", too.