What seems most insidious is that the tasks that lead you down the rabbit hole make you feel like you are actually being productive, which is probably why it is so hard to intrinsically determine when you are heading down that hole in the first place.

I have noticed that most of these tasks relate to web-browsing and have experimented with just turning off my wireless while I am working. This usually means that when I need to do something "unproductive", I need to make a little bit of an extra effort to get online, which is generally enough to think to myself, "Hey, I should really get back to coding..."