Not Keeping Score
Keeping track of personal bests might sound like a diversion for fitness wonks, but there's a reason why pro coaching outfits such as Athletes' Performance do it: If you want to improve your fitness, you have to track your training. Exercise logs are like job evaluations—without them, you don't work as hard.

The Fix: Keep a training log. If you want to get obsessive about it, Web sites like Trainingpeaks.com, which tracks performance metrics such as speed and power output on bike rides, are great. Otherwise, a heart-rate monitor will do just fine. You ran four miles in 35:15, with a heart rate of 144 beats per minute yesterday? Match those numbers—or beat them—the next time you go out.