What's the best way to get a flatter stomach?

The best way to get a flatter stomach is an exercise I call table push-backs. That is, push baaack from the table and stop stuffing your face. Contrary to the latest infomercial, you won't get flat abs by using the latest and greatest ab contraption. The secret to flat abs is your diet. That right: It's 80 percent diet and 20 percent exercise.
You can do a million crunches until you're blue in the face and that lovely muffin top will remain covering your toned abs until you change your diet and start to eat clean. A clean diet means eat everything closest to its natural state (so skip commercial snack foods...donuts, cakes, cookies, chips--even the organic ones, processed foods, high-fat salad dressings, all fried foods, fast foods, alcohol, soda, sugary juice...you get the idea). Eating a quality diet of fresh vegetables, fruits, whole grains, and low-fat proteins (in the appropriate amounts) is the best way to get better abs and a better body overall.
As far as exercise goes, cardio is primarily for toning the heart muscle and burning fat; strength training is for toning muscle (through body weight exercises, free weight training, kettle bells, weight machines). Get in a good cardio workout most days a week, and make sure to do toning exercises that also target your abs. Try to do toning exercises at least two to three times a week on non-consecutive days.
Here are some good ab-toning exercises you can do at home (do what you can, but try to work your way up to doing more reps and holding the positions for longer amounts of time):
Plank: Lie on your stomach on a mat. Then get into position by placing your forearms onto the mat and lifting up so that your shoulders are right over your elbows. Then extend your legs behind you and rest your weight on the tip of your toes (like if you were doing a pushup) and your forearms. Make your body from your shoulders to your toes a straight line (don't lift your hips too high or arch your back). Keep your abs tight and hold this position as long as you can without bending your back. Try to hold it at least 10 seconds, and gradually work your way up to holding it for 60 to 90 seconds. Take a break and then repeat two more times, for a total of three sets.
Side Plank: This is similar to the regular plank, but done on your side. So get on your right side, with your legs straight and stacked one on top of the other. Extend your right arm so that you are propped up on your elbow, which should be lined up under your shoulder. Put your left hand on your left hip. Keep your body straight, not allowing your hips to dip towards the ground. Keep your breathing steady. Hold this position for at least 10 seconds (try to work up to 90 seconds eventually). Switch sides and repeat. Do three sets on each side.
Leg Raises: Lie flat on your back, and extend both legs toward the ceiling with your feet flexed (as if you were standing on the ceiling). Slowly lower both flexed legs toward the floor, and once they get about six inches from the floor, slowly raise them back up to starting position. Place your hands under your buttocks for extra spinal support. Do three sets of 10 to 25 reps.
Results will vary from person to person, but if you follow these guidelines, most people will see visible improvements within a month, no matter what their fitness level.