Believe it or not, I keep a stop watch on my desk. It's right there by my computer. It's digital with 2 buttons: start and stop/clear. And it has proven to be such a useful device to help improve my focus.
When I have a repetitive task that I have to do (one that I can't farm out or outsource), I time myself the first time through. Then I reset and I do the next task and try to beat my last time. It's a fun way to get though the project quickly.
On larger jobs, I often write an amount of time I anticipate the job to take right in my planner and I time myself as I work to help keep me on task.
Get yourself a stopwatch or a watch with a timer and pay attention to the time you spend on projects. If you time each project, you'll be amazed at how your focus improves.