I have attention span problems too, but I found a technique that helped me. It's called Pomodoro Technique. From their site:
1. Choose a task to be accomplished
2. Set the Pomodoro to 25 minutes (the Pomodoro is the timer)
3. Work on the task until the Pomodoro rings, then put a check on your sheet of paper
4. Take a short break (5 minutes is OK)
5. Every 4 Pomodoros take a longer break
I use this free adobe air app to track my time. PomoDo
I thinks it's a very easy and useful technique, specially because you can raise from your chair and walk or stretch out a bit on the break, so you won't be sitting for too long. I usually take more than 4 "pomodos" before the longer break.