C.O.
I just flat out told my son that the only people who were allowed to see his private area, defined as the area covered by underware, was mommy, daddy and the doctor with mommy or daddy there. I also told him that if anyone every touched him there without his permission, to tell a trusted adult...even if he was telling on mommy or daddy. We started this around 3 years of age.
Then we role played, like we do for drugs, cigs, doing mean things to others, and yes, the bad touch... I would ask him what he would do if someone wanted to see his private areas and his stock answer became "No, that's private! then I would run, run, run and tell an adult!"
I ask him every so often, and have had to give reminders now and then and re-exlpain, or as he has gotten older, flesh out the explanation a little bit.