A few times recently, I've overslept a lot (till 11, when I normally wake up 9:30 at the latest). When I have the chance to sleep in, my sleep schedule gets all wierd, I wake up a lot, and tend to oversleep. If you find that you wake up a fair amount, try figuring out how often you wake up, and then don't go back to sleep if you're dangerously close (within less than one sleep period) of when you would wake up. Alternately, set your alarm for early morning, and drink a lot of water. That should get you up.

In general, I find that the more I concentrate on dreaming and sleeping comfortably, the more I sleep in.