Found this on parenthacks:

I got tired of being the boss of when my kids (three and five) could watch videos and DVDs. I also wanted to help them learn to make choices about media consumption while they’re still young. So I decided to set up a ticket system.

Every Friday, they each get tickets (purple for one kid, green for the other) that can be redeemed for ½ hour of TV time.

See full article. And this post helpfully gives designs so that you can create the tickets yourself on your printer.

I am tempted to try it out, but I think that my kids might be a little too young for this.