Diet. Above all else, diet is what works best. There are various regimens to consider, from low fat diets to low carb diets - find which works for you not only in the short term but something that you can continue doing so you do not regain weight. Speak to a nutritionist to help you establish a good diet.
Exercise too. Proper diet and exercise, good sleep patterns, self control, reducing stress, having these and some accountability. It's a lifestyle change and commitment, not a quick fix.