The abdominal muscles are some of the trickiest muscles to develop. Getting a six-pack takes a lot of time and dedication - and the right exercises. Here are three of my favorite ab-exercises:

Leg raises: This is probably the best exercise for developing great abs. It engages most of the muscles in your stomach area, and it's great for working out the lower abdominal muscles - a muscle group many people neglect when they're working out.

Lie flat on your back, legs straight, hands by your side. Lift your legs - while keeping them straight - until your body forms an "L", creating a 90 degree angle at the hips. Lower your legs back down, until they almost touch the floor. Then lift back up. Exhale as you lift, inhale as you lower.
A variation for a greater challenge and faster results: Place a medicine ball between your feet, and lift it up and down as well.

Crunches: The all-time favorite ab-exercise. Lie on your back, feet flat on the floor, knees bent. Place your hands behind your head, and point your elbows out to the sides. Lift your upper body, engaging your upper abdominal muscles. You only have to lift your head about ten inches off the ground. Make sure not to move your elbows inwards, and don't tuck your chin. Lower back down.
Again, exhale as you lift, and inhale as you lower.
A variation for a greater challenge and faster results: Instead of placing your hands behind your head, hold a dumbbell in front of your forehead or chest, as you do your crunches.

Sit-ups with a twist: Position yourself as you did for crunches, legs a little straighter though. With the hands behind your head, lift yourself up to an almost seated position. Again, avoid moving your elbows inwards and don't tuck the chin too much. As you reach the almost seated position, twist your upper body to the right. Lower back down. Repeat, this time twisting to the left. Exhale as you lift, squeezing the last bit of air out of your lungs as you twist. Inhale on your way back down.
I like finishing up with a thirty-second plank. For this, place yourself in push-up position, then lower your elbows to the ground, hands pointing straight forward. Make fists for better support. Keep your body straight, and breathe calmly. Hold this position for a slow count to thirty. The plank is a great exercise for core strength.
After an ab-workout, make sure to do back-exercises as well, to keep your body balanced.