As usual, there is a great advice on the subject from Terence Tao. See also a paper in the Science Careers. The comments with further suggestions and links are welcome!
Another useful tip from the Lifelong Scholar’s blog: whenever you take a break, make you sure you have a specific task to do when you get back to work.
Update: there are many more resources on the academic productivity. To list a few,
Stumble!
Researcher's Blog 