You Can Do Anything In 15 Minutes
One of the seven key characteristics of my workouts is that they have time limits. Further, each exercise in each workout also has a time limit.
Check out THIS and THIS: two great posts on how you can boost personal productivity by using a kitchen timer.
I'm not sure what it is about 15 minutes (I'm always asked "How about 20 minutes?"), but it seems to hit the sweet spot between "long enough to get something done" and "short enough to be tolerable."
Choose whatever time interval you like, but if you're a procrastinator, or find yourself easily distracted, try putting a clock on yourself.
BONUS: Egg timer widget for Mac dashboards!

Escalating Density Training DVD
Muscle Logic
The Unnatural Athlete
Physically Incorrect 
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