I'd say a good rule of thumb is "don't trust anything an infomercial says".
You can do 5,000 crunches a day, but if you don't burn the fat on top of your abs, all your work will be wasted. I've learned the hard way that there are no shortcuts to a nicely toned abdominal area. As Emily said, you can't spot reduce fat.
First and most importantly, you need to get your diet in order. That means a lean diet high in protein and low in carbs. Avoid things like high fructose corn syrup, hydrogenated oils, enriched flours, and trans fats. Avoid dining out unless you really know what you're doing. If you must dine out, get something on the "grilled" menu, preferrably chicken or fish (salmon is my favorite).
Second, (you know what's coming) ... Exercise!!!
Start with 20 to 60 minutes of your cardio machine of choice, 3-6 times per week (the more the better). After that's done, focus on the big muscles. Squats, dead lifts, bench press, and some bicep/tricep workouts every now and then as well. You only actually need to focus on abs 1 or 2 times a week if you're working out the rest of your body.
Muscle anywhere burns fat everywhere.