Good answer. I agree, eating is key. Up to about 25, you can probably pretty much eat what you want. After that, you have to start keeping track of what you eat. Being active is the second most important thing; it's not easy at first, but it gets easier if you keep doing it. If you're like me, some kind of positive reinforcement is extremely helpful; I'm using Wii Fit to keep track of my weight and BMI. That way, exercising and being healthy starts to feel like playing a game because you can see your progress, and you can immediately see when you start to slack off.
Appearently they did a study on how precise it is, "Comparison of Circumference-and Skinfold-Based Body Fat Estimation Equations", where they compared their formula with several other methods for body fat estimation, against a baseline set by hydrostatic weighting. I couldn't find the full text online (at least not for free), but the abstract is here:
Quote: "For men, the Navy equation was the best predictor of body fat determined by hydrostatic weighing."
So all you need to track your body fat is a measuring tape plus a pen and a pad. That's too much of an effort for me, though, so I use one of the web-based calculators, e.g.
