The abdominal muscles are essential in most skills, and if they're not directly involved, abdominal strength is usually very helpful.
Body Tighteners
Begin by lying on the floor on your back, with your arms above your head and legs straight out. Then pull your arms down by your side and lift your legs an inch or two above the ground. Your back should be the only part of your body touching the floor, and your lower back should be rounded and in contact with the floor. Do not let your lower back arch and come up off the floor; this would place harmful stress on your back.
Hold this position for a set amount of time (between 15 and 30 seconds). Rest and repeat for the same number of seconds as before.
V-ups
Begin in a body tightener position, but with your arms above your head. Then using your stomach muscles, bend at the hips and bring your chest and legs up simultaneously into a V-position. Drop your arms in front of you. Then drop back into a body tightener, but do not let your arms or legs touch the ground. Repeat.
Tuck-ups
Begin in a body tightener position with your arms above your head. Use your stomach muscles to lift your chest and pull your knees to your chest simultaneously. Drop back into a body tightener, without touching your arms or legs to the ground. Repeat.
Hollow-body rocks
With your body in a hollow-body position, with your arms squeezing by your ears, rock back and forth on your back, only letting your arms and legs coming about six inches from the ground. Throughout these repetitions, your body should remain hollow and tight.
Hip rises (or "butt-ups")
Begin by lying on your back, your arms out to the side on the floor, and your legs elevated about 90 degrees from the ground. Using as few other muscles besides your lower abdominal muscles, lift your hips up off the floor only a few inches. Then roll your hips back on the floor and repeat.
Leg Lifts
This exercise can be done in both pike and straddle position. Place your hands on the floor next to your knees, and then scoot them forward three inches. Leaving your hands in this position, lift your legs off the ground about three inches. Drop your legs, but do not let them touch the ground, and repeat.
Leg Lifts on the Bar
Begin by hanging on the high bar. Without swinging, lift your legs straight until your toes touch the bar. Slowly lower your legs back down to a hanging position and repeat.