I find that when I have something difficult, or new, to do I want to read HN and other sites more frequently. However, once I start on those hard / new tasks they are invariably engaging enough to keep me focused. It's just that initial hump.
[1] http://blog.xkcd.com/2011/02/18/distraction-affliction-corre... This works surprisingly well
The approach I'm experimenting with is a "productivity mode" toggle in a tiling windows manager which disables most of the windows manager commands, leaving me mostly stuck in whatever application(s) were already visible until I toggle it off. The toggle off function has a 30 second delay.