When I arrived at work this morning, I had 4 things that all had to be done fairly urgently: create a storyboard for a video to be reviewed at a meeting tomorrow morning, revise some documentation before sending it to a developer for review, provide some documentation effort estimates for the next release, and deal with some online Help bugs. My problem is that it would be fairly easy to spend an entire day doing any of these, and I can't decide which to do first.
Fortunately, I keep a kitchen timer on my desk. This morning, I set it for one hour and started in on the storyboard. At the end of the hour, I had a rough storyboard complete. Given my perfectionism, I could easily have spent another hour tweaking and polishing, but since my timer was up, I just sent it out to the meeting attendees.
Then I set my timer for another hour and dove into the documentation estimates. When that hour's up (I've paused the timer to compose this post), I will decide whether I want to spend another hour on the estimates, or switch to bug fixing.
You'd be surprised how much you can accomplish in just an hour if you don't allow yourself to spend any of that hour worrying about whether you should be doing something else instead. (Or worse, trying to do something else at the same time.)