Depending on your age, this is likely something your parents didn't have to deal with as much as parents of today. There's no going back to the days when the only worry was what was on the radio. Preteens have access to a number of entertainment media: television, video games, the Internet, music and movies.
A common suggestion among professionals is to monitor and moderate media exposure for your preteen. This means understanding the ratings systems -- such as the Entertainment Software Rating Board (ESRB) for video games and the MPAA for movies -- in addition to keeping an eye on their media consumption. Many parents use a rewards system to keep track of how much their child uses such media. For instance, they may decide their child can play two hours of video games for every three hours he does homework.
It's important to find a system and stick with it across the board -- no exceptions for the finale of their favorite show or if they get to the top level of a video game at the end of their time limit. Standing by these rules establishes your boundaries and also teaches good time management skills to preteens.