6. No Crash Dieting
Though your slow metabolism might be reason for your weight gain in the first place, one of the worst things you can do is to crash diet.
By drastically lowering your food intake, your body will think something is seriously wrong and it will go in to what can be described as survival mode, which means that it will slow down your metabolism completely because it thinks you are being starved.
This is opposite of what you want to happen!