In an article called How the Web and the Weblog have changed Writing, Philip Greenspun writes:

In the 1980s Steve Ward, a professor of electrical engineering and computer science at MIT, described a sure-fire dieting scheme. "All that you need for my diet is graph paper, a ruler, and a pencil," Steve would explain. "The horizontal axis is time, one line per day. The vertical axis is weight in lbs. You plot your current weight on the left side of the paper. You plot your desired weight on a desired date towards the right side, making sure that you've left the correct number of lines in between (one per day). You draw a line from the current weight/date to the desired weight/date. Every morning you weigh yourself and plot the result. If the point is below the line, you eat whatever you want all day. If the point is above the line, you eat nothing but broccoli or some other low-calorie food."

Steve noted that this could also be called the "Bang-Bang Servo Diet" but that would likely be confusing to non-engineers (see http://en.wikipedia.org/wiki/Bang-bang_control).

Steve's diet is probably more effective than most popular diets. How come he isn't a bestselling diet book author? How do you turn an idea that can be explained in one paragraph into a diet book that people will buy?

I also wonder: has someone created a web site where people can implement this diet without needing even the graph paper and pencil? It could be sponsored by one of the bathroom scale manufacturers. People could choose to make their charts public and give each other moral support. The web site could send automatic email feedback — congratulations when you've hit a milestone, encouragement and suggestions if it detects you've been slipping, and gentle nudging if you haven't updated the chart in a while.

Seems to me this would also be a natural idea for a desktop and/or iPhone app, with or without some sort of online aspect. I wouldn't be surprised if somebody's done it already, but I'm too lazy to search the App Store right now.

P.S. One reason I thought of the email reminders was something I saw on The Skeptical Hypochondriac:

A recent study found that email reminders can make a huge difference on the amount of physical exercise done by recipients.