Start the next task before you mark off this one
This is a time management technique related to Stop in The Middle: if you're having trouble coming back to your work after breaks, or if you have trouble moving onto the next task after you finish the first one, try this trick:
After you finish a task, don't check it off/cross it out/mark it done. First, look at the next task on your list, and do the first step of that task. It doesn't have to be much, just whatever you have to do first to begin the next task -- maybe something like pulling out the file for the next project, or booting up the computer, or starting water running in the sink. THEN you can mark off the previous task.
You are usually willing to do that first step, because you know that you can take a break as soon as that step is done. But you'll often find yourself thinking, "Well, I've got it started, I might as well finish it." I've gone down entire to-do lists in this fashion without a break, because just getting started is often the hardest part of the whole thing, and anything that gets you over that hump can make you enormously more productive.
Doesn't sound like you? Go back to Time Management Hacks and look for a time management technique that fits you better.