I have done loads of dieting over the years. For me, the answer was simple - either cut down on your carbs (energy source), or make sure your exercise output is greater than your energy input.

I lost loads of weight previously really quickly doing the Atkins diet (cutting out carbs almost altogether), but this is not healthy. Now, I am just cutting down on carbs, so its more of a lifestyle change than a diet. The weight is coming off really slowly, but, because I just adjusted my way of eating, I believe it will stay off, whereas with an extreme diet, you lose the weight quickly, but then put it back on again when you return to more normal eating.