There are lots of great suggestions here. It depends very much on each person I guess but every method is worth trying.
This morning actually I was about to stay in bed for longer when the clock rang. But I thought: how will you achieve anything today if you sleep? I thought again of the goal I have set to achieve by mid-2007 and got up straight away.
It's normal to have low-productivity days. The consolation is that after one of those, you realise how little you have achieved and that can work as a great incentive to do more to compensate.