A good, daresay "classic" article on motivation and productivity, from Joel Spolsky:
Fire And Motion - Joel on Software
Also, there is always the possibility that there is a deeper issue at hand: whether or not you actually like what you're doing. I tend to get that "big productivity, big crash afterwards" when I am working too hard at doing something I don't really like, but "have to" do.
A related article on work + passion by Paul Graham...
How to Do What You Love