A delightful resource to help children monitor their viewing is The TV Smart Book for Kids by Peggy Charen. Ask you children to write a report card for each show watched. Have them rate it according to violence, values, and content. Encourage your children to become critical viewers and to watch only the best programming available. Don't use TV as a babysitter.
TV is a poor substitute for human companionship.
I have been guilty of allowing my kids to watch TV to keep them occupied when I needed uninterrupted time to get something done. Be a good role model. Instill good habits by being a good example. Make the TV inconvenient. One family didn't replace their antenna when it was blown over in a windstorm.