Parental Controls
Parental controls aren't just for parents anymore. World of Warcraft comes with a very useful tool for managing playtime. Using what it calls a 'Play Schedule' you can set times during which the account is available for play. A picture of the play schedule is below:
The red boxes are times in which the user is unable to play, and the green boxes times when the user is able to play. You can pick from premade schedules that include 'All red', 'After school and weekends', 'Weekends only', and others. You can also set your own schedule by clicking each little box to toggle it between red/green.
And they're not just for parents; many players give their spouses control of their playtime to help achieve good balance. A determined player is probably crafty enough to bypass this control, but it is still a decent deterrent.
