I find that when I'm working on hourly-billable projects (in particular, those that are research/design/architecture-oriented as opposed to straight coding) that I'm easily distracted by any number of things (email, grab a drink (loss of focus, but nature happens), link off the webpage I was reading, wandering mind (easy when the job calls for a lot of thinking), etc.) This results in very fragmented time, far too incremental IMO to accurately track with a timeclock, and some time very gray. I frequently end up billing for only some fraction of the elapsed time I spent in order to feel fair, but sometimes it takes a really long time to put in an 8-hour day.

By contrast, when I've worked for salary I've not worried about whether I'm actively working at any given minute, I just get the job done, and I've never had anything but stellar reviews/feedback from past salaried employers, so I think I get the job done well. I personally believe in an 80/20 cycle: I get 80% of my work done during an inspired 20% of my time. But I have to screw around the other 80% of the time in order to get that first 20%.

So the question: what billing/time-tracking policy can I adopt in order to be fair to my hourly customers without having to write off my own less-productive 80% that a salaried employer is willing to overlook in light of the complete package?

Note: This question is not about how to be more productive or focused. It's about how to work around whatever salient limitations that I have in a way that's both fair to me and to my customers.

Update: A little clarification (to pre-emptively stop some righteous indignation): I currently have a half dozen different project/client groups. It's not a great situation and I'm working at reducing it down to two, but that's my current reality. It's very easy to get off on a thread related to a different project than the one I'm clocking, and I'm not always conscious of it at the time. [I did not intend the question to mean that I was off playing games or making personal calls, etc., and have adjusted wording above to be clearer. Most of the time. I am only human, and sometimes the mind does force you to take a break! :-)]