I've had the same problem. To solve it, I bought a 24-hour timer switch like this one: http://www.cnaweb.com/productimages/215030.jpg, and plugged it in near my bed. To that I hooked up a cheap garden water pump. Next, I cut out the back of a cardboard box and taped it over the timer with lots and lots of duct tape. The box has a lid so that I can adjust everything as needed.

Now, when I need to wake up at a certain time, I set the timer accordingly, lock the cardboard box lid shut with a padlock, and place the key somewhere outside.

In the morning, the water wakes me up and I can't go back to sleep without turning it off. To do that I need to get up and go outside to get the key, and by the time I've returned, I'm awake enough to make a rational decision to stay awake.