Do Crunches. Lie on the floor (with or without a mat). Cross your arms in front of your chest(Do not place both hands behind your head-- Placing both hands behind your head can cause lower back problems in the long run as it places unnecessary stress on that part of your body from pulling on your head and neck.) Another alternative to crossing your arms in front of your chest is to lightly place your finger tips behind your ears, without pulling on your neck or ears to help raise you off the floor. Draw in your abdomen to your spine while inhaling through your nose. Now raise your shoulders (upper torso) towards your knees, using strictly your abdominal muscles. It is very important not to lift your entire back off the floor, as this can cause back strain, and the extended movement does not help you develop six pack abs any faster. The most important part of the crunch is the initial flexing of your abs as you lift your shoulders off of the floor. As soon as you begin lifting off the floor exhale through your mouth, ending with a gasp once your shoulders are off the floor. Then pause for a second once you are at the top of the crunch and exhale the last bit of air from your diaphragm while flexing your abs. The proper breathing and flexing make all the difference. Now lower back down slowly and controlled while inhaling through your nose, just until your shoulder blades touch the ground. Do not let your head touch the ground and repeat
Do sit ups. Lie on the floor, feet on the floor, knees up and hands behind your head or crossed on your chest. Sit all the way up, lifting your lower back off the floor along with your shoulder blades. Lower yourself down. Repeat. Once this becomes relatively easy for you (i.e. you can do a quite a bit with ease) start adding more challenges. Find an incline bench or do these on an exercise ball. Once you "graduate" from that, do weighted sit ups. Hold a weight on your chest while you do these. As these become easier, hold heavier and heavier weights. You might also try lifting your feet off the ground while doing the sit ups or alternating the leg in the air, like pretending to pedal while sitting up. If your hands are behind your head, bring your left knee up to touch your right elbow and then your right knee to the left elbow.
Do Leg Lifts. Lie on the floor, legs straight out, hands at your sides. Lift your legs straight up (not bending your knees at all) until they're at a ninety degree angle (or close). Lower your legs and repeat without letting your legs touch the floor. For more challenge there is equipment at most gyms that will allow you to raise yourself up using your arms as support and dangle your legs. You can perform leg lifts there too. If you're using this piece of equipment, you can make it easier by just raising your knees to your chest. It's more difficult to raise your legs to a horizontal position with your legs straight. This helps firm up the lower abdomen. If you're truly a monster, try doing leg lifts with a medicine ball hanging from your feet. Or hang from a pull up bar and raise your legs in front of you all the way up to the bar. Still too easy?
Do Jackknife Sit Ups. Lie down flat on the floor. Place your hands on the ground to your sides for balance; you can pick them up as you get used to the movement. Simultaneously raise your knees and torso so that your knees and face meet on an imaginary line extending from your pelvis to the ceiling. You should be able to kiss your knees at the top of the motion. Your legs will naturally fold bringing your feet towards you hips, much like a jackknife. Lie back down (i.e. "spread out") and repeat. Place a weight between your feet when you think you can handle it. What's that? More?
Do V-ups. Lie on the floor, legs straight out, hands on the floor but this time extended out over your head. Simultaneously raise legs and torso. Don't bend your knees! What kind of V would it be if you bend your knees? Reach with your hands toward the raised feet. Touch your feet if possible (might require some flexibility). Relax, return to starting position and repeat. Add weight between feet to match your taste.
Train Your Oblique Muscles. It's not as important to work on your oblique muscles at first, but eventually you'll want to start working these too. These are the muscles to either side of your stomach. There are multiple ways to do this and anything that includes twisting your torso against a resistance counts. There are twisting machines at gyms, you can twist while you do sit-ups, you can do side bends, you can twist side to side with a medicine ball in hand, etc. Be aware though, that many beginners tend to have weak obliques compared to their abs (it simply isn't used as much in daily life) so go easy on the sides at first
Other stuff Since it is, literally, the center of your body there are many other tricks you can use to train your abs, and some of the other movements will more or less involve your abs. Including every abdominal exercise in existence would make this article painfully long and new methods are being developed constantly. Now that you've made up your mind about a washboard mid-section, get creative! Find new ways to crunch, bend and twist in your daily life. Some possibilities include:
Use a stability ball. Do your crunches on the ball to introduce instability to your workout, which will improve your balance too. There are also lots of core exercises that can be done with a stability ball.
Duck and twist during your daily routine. Reach with your left hand to things on your right and visa versa. If you feel like turning around to face something, see if you can do it with keeping your hips in place and twisting at the waist (warning: awkward when talking to other people, use only against inanimate objects). While walking or standing, pretend that something is coming toward you and you have to duck to get out of the way. Do this as often as you are comfortable or at times when it won't look weird. You can bend forward from the hips or, if you're really into it, bend at the knees too and really "sink" out of the way.