Sadly, there's no easy way to target belly fat -- in fact, the midsection is usually the last place from which fat disappears. You have to reduce your overall percentage of body fat for this change to occur (note: even if you're "slim," you may still have a high percentage of body fat).
Too much cardio may actually cannibalize your muscle (and lower your resting metabolism) before it burns your belly fat. This is one reason why you see so many runners with emaciated limbs and pot bellies!
What has worked for me is a program of:
1. Exercise combining weights, high intensity cardio, and moderate intensity cardio (for recovery)
2. NO processed foods, being especially careful to avoid white flour, refined sugar, alcohol, and additives
3. A high fiber diet, which fills me up without adding lots of calories
However, there are some simple things you can do to speed things along.
Green tea/white tea can help burn belly fat, as shown in recent studies, and 1 to 2 teaspoons of apple cider vinegar can help metabolize fat as well. Hot peppers in food can boost metabolism by as much as 25%, and almonds, in small quantities, have also been shown to reduce body fat (I EMPHASIZE: IN SMALL QUANTITIES). Taking in a cup or two of coffee or strong tea an an hour before your workout will help you work out more efficiently and give you a better metabolic boost.