Abdominals
Your abdominals (stomach muscles), are one of your more important muscles groups, especially for martial artists. These muscles are the physical center of your body, and therefore should be taken care of. Here I have a few exercises that are very helpful to strengthen and shape these muscles.
Sit-Ups
The very root of all stomach exercises, sit-ups are the most important. These can be done anytime, anywhere with a minimal amount of floor space. Lay on the floor and bring your feet in close to you. Then simply "sit up" all the way. There are many variations to this. You can either cross your arms over your chest when doing these, or interlace your fingers behind your neck, sit up, and turn, touching you right arm to your left knee, and vice-versa. Make sure you don't put your hands behind your head. This may result in a neck injury. Then, there is also the stomach crunch, which only requires you to lift off the ground several inches. This is good if you want to focus more on your stomach muscles, and not on your back, which the full sit-up utilizes. I would suggest for beginners to go through these exercises first to find out how much they can handle, and which one works best for them. From there, simply workout a goal each time you use these methods. I would further recommend using a slant board for sit-ups. This gets the job done much faster, though they are much more difficult to do. When using one of these, you should use the sit-up and turn method I explained above. Doing any of these exercises more slowly will work more on strengthening the muscle groups, while working faster will increase endurance.
Leg Raises
Leg raises mainly work your lower stomach muscles, and also work your lower back . If you've never done these before, you may find them quite difficult, especially if you have not done any other stomach exercise. Take your time. Simply lie flat on your back, your legs extended. With your legs pressed together, lift them a off the ground until they reach about a 30 degree angle with the floor, then set them back down. Do as many of these as you can.
Frog Kicks
A rather uncommon stomach exercise, frog kicks work your upper stomach muscles. These can be done if you have a chin-up bar handy. However, the counter top in my kitchen has an "L" shape corner to it, where I can hold myself off the ground. However you manage to do this, hold your legs together, and bring your knees up to touch your chest. Whatever you do to hold yourself off the ground, make sure it's sturdy!
Twists
These are a pretty good cool down stomach exercise. You need only a stick of some sort, such as a broom handle, or a bo if you happen to have one. Rest it across your shoulders behind your head, and hold onto each end. Next, simply twist to the left, touching the right side of the stick/broom handle to your left foot, and vice-versa. These are not very strenuous at all, so I would recommend you do at least 50 (one repetition would be twisting to the left, then to the right).
...Jedi Streen