I agree with Dragonchilde. Core exercises (including sit-ups and crunches) do NOT burn stomach fat. Energy is delivered to the muscles via the bloodstream, NOT from being absorbed from surrounding fat stores, and unfortunately this means you cannot target where you lose fat from.
What you can do is to continue to reduce your overall body fat through a combination of cardio, all-body strength training, and watching your intake, and some of it will naturally come off the stomach.
However, a strong core is important for posture and balance, and planks are some of the best core exercises going around. Demo at www.sparkpeople.com/resource/exercise_demo