A good balance that I have been trying to achieve is working my tickets, and when I start to really feel drained from that in the day I take a break. This break, can be a short walk, or a longer "study session" in which I crack open the technical book that I am working through at the moment (just finished the Pragmatic Programmer).

Some folks may call the study session slacking, but I have made it a point to work in places that say they want that sort of self directed growth, so I don't sweat it.