Try the Pomodoro Technique. Give yourself 25 minutes to focus, and absolutely force yourself not to do anything else but the task at hand. No exceptions. When the 25 minutes are up, you have 5 minutes to relax and unwind. Force yourself to do anything but work for these 5 minutes. I'd strongly recommend leaving your computer/workspace and walking around rather than browsing the internet, but to each their own. Again, no exceptions on the not working part though. When the 5 minutes are up, get back to work.
Adjust the time intervals as appropriate. I tend to prefer 45/10 but other days I do the original 25/5.
There are plenty of pomodoro timer applications for every OS. My favorite for windows is pomodairo: http://code.google.com/p/pomodairo/ and I can't recall the name of my Mac OS X timer at the moment, but I'm sure you'll find it with a little searching.
Here is the original website for the technique: http://www.pomodorotechnique.com/
