One way of doing this is to eat carbohydrates all the time without eating much in the way of protein and fat while doing some kind of exercise that involves moving your body around such as cycling. That way your body wants to be lean to do the exercise and figures it doesn't need to make and store fat because it has a ready energy source in the form of the carbs coming in all the time.
This is all well and good if you want to be clinically lean, but most of us want to be muscular as well. The holy grail would be to have lots of muscle and no fat without having to use any brute force methods such as calorie restriction or stimulants. My protocol of bodyweight resistance training, whole body exercises and workouts several times a week combined with separating carbs and fat comes the closest to this I've seen, but it still needs refining.
The approach I'm taking is to think that if I were a body, what would I do to it to make it think "Aha, I really need to get rid of this fat and gain muscle to withstand this". Does anyone have any ideas?