Regularly weighing yourself can motivate you to shed excess pounds. Dieters who took action when the scale climbed more than 3 pounds were 82 percent more likely to keep weight off than those who were less vigilant, according to a study published in the New England Journal of Medicine in 2006.

But the scale won't tell you much about your health. For that, the best indicator is turning out to be your waist circumference, which indicates abdominal obesity. That's important, since belly fat is more metabolically active than fat stored in the hips and thighs and is a strong signal of increased risk of disease.

In April, the two largest studies of abdominal obesity—one a nine-year follow-up of about 250,000 middle-aged men and women surveyed by AARP, and the other an analysis of 16 years of data from more than 44,000 women in the ongoing Nurses' Health Study—published remarkably similar findings. Both found that people with a larger waist circumference were more likely to die prematurely than those with smaller waists, regardless of their body mass index, or BMI.

The study also found a strong association between early death and a large waist-to-hip ratio—a measurement of the narrowest part of the waist compared with the circumference at the broadest part of the hip. Although the waist-to-hip ratio can more precisely differentiate belly fat from hip and bottom fat, it requires two measurements and so is more prone to error and less convenient to calculate.