I like the graphics on a page from Tulsa World of Gymnastics. Here’s a sample:
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.
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.
See the rest of the article – Strength and Conditioning: Abdominals
They provide plenty more articles like this in their Gymnastics Tips for Gymnasts and Coaches section.
(via About.com: Gymnastics)