Fat Burning Heart Rate
The convnetional wisdom dictates that you should exercise at a level between 65 & 75% of your max heart rate for the best fat burning heart rate. This is fine and will burn off fat, but it will also burn off muscle too, which is bad.
Why is it bad? It's bad, because the less muscle you have on your body, the lower your metabolism will be, which means you will burn less energy when resting. To combat this, you will need to do weight training and eat more protein to ensure the muscle doesn't atrophy.
There is a movement in fitness circles and even research done in Australia that is starting to prove that the most effective fat burning heart rate is near your maximum, but only for short intervals, intersperced with lower intensity training. This new form of workout is call high intensity interval training, and works by quickly switching between high intensity intervals and low intensity recovery periods.
The best results are found when a period of HIIT is done for twenty minutes, three times a week. The reason this training method works so well is that it doesn't burn off the muscle, unlike lower intensity training, and because the intensity level is higher, it raises the metabolism more when resting.