That's a really interesting question. What I've found is that when I'm obsessed with a project it tends to almost always be on my mind, so that when I sit down to work on it I generally have a very good idea of what I want to accomplish and how to go about it (or at least how to get started on it). And when I know I'm only going to have a limited amount of time to make progress then I really try to do as much mental prep as possible so that when I do sit down I can really do some damage. That said, there are times when I end up getting sucked into reading HN and accomplish very little. ;(

By the way, one quick trick I use to avoid wasting too much time on HN is to use the Instapaper bookmarklet to print out the articles in a condensed format to read later when I'm away from the computer.