Diet and proper nutrition. Figure out your daily calorie needs and then go deficit by 500 every day. One trick is actually to multiply your body weight by 10, 11, or 12, depending on your metabolism. If you have a high metabolism, multiply by 12. For a low metabolism, multiply by 10. Most have a medium metabolism and should multiply by 11. The number you get is an approximation for how many calories you should eat daily. So if you weigh 200lbs, and you have a medium metabolism, you'll get 2,200.
If you work out regularly, be sure to get plenty of carbohydrates. Also, spreads meals throughout the day. I usually have six meals daily, with one of the mid day meals being large. I learned to be sure to get enough calories earlier in the day so I don't feel so starved at night.
If you are looking for exercise to do the trick, use less weight but more reps. Also, be sure to do full body workouts. The larger the muscle, the more fat gets burned. Ab machines are worthless. The leg muscles bun the most calories just to sustain them, and working them hard leads to a need for repair. Combine weight lifting with a cardio workout.
Hope this helps!