I don't maintain focus for more than an hour at a time. In fact, I can't. And being involved in cognitive science research, I can state that for sustained work, neither can anybody else. Yes, once in a while, if your current problem is intensely interesting, you can zone out and work on it for many hours straight. But most work - however interesting it is - isn't able to grab your focus in that manner. To see it from another perspective, a movie is designed to grab your attention, is created by people devoting their lives to grab your attention as effectively as possible and has about every technical means short of drugs available to grab and hold your attention. And yet, few movies are longer than two hours, because people will not be able to hold their focus for much longer than that.

If you try to force yourself by having only your work available and forcing yourself to sit on front of it, all that will happen is that after an hour or two your thoughts will start to drift, you'll get stuck in a rut reiterating old thought processes, and you may even nod off for short periods due to the imposed lack of stimulus variation. In short, "maintaining focus" is a good way to sink your productivity.

Instead, accept that you can't single-mindedly focus on any one thing for more than about 45 minutes to an hour (there's a good reason class time seems to have converged to about 45 minutes the world over). Do one of these things in combination:

* Get up, stretch and move about a bit. Go look out the window, find someone else on a break and shoot the breeze for a few minutes. Leaf through the morning paper, trade magazine, or that 2001 office supplies catalogue you never seem to get around to throw away. Go over to Accounting and ask about that missing trsvel reimbursement. In short, get yourself exposed to some new stimuli so you canh approach the next hour with a fresh mind.

* Have a secondary task you can switch to whenever you get bored with the main one. Maybe now is a good time to answer a few email, or write some documentation (there's always documentation to write). Really the same thing as above - get some new stimuli - but with more of a work focus.

* Set a goal for the day. And when it's done, quit. Sure there's more work to be done - but there's always more work to be done. You can stay 24/7 and there will still be more. Do what you set out to do, then go home. With a definite short-term goal, pacing yourself is easier, and it's much easier to focus on a comprehensible, digestible chunk of work.