If you want washboard six pack abs, you don't need to train them directly. There are actually exercises that train your abs better than any variation of sit-ups or crunches. This is a killer exercise to develop washboard abs:

Explosive Medicine Ball Pushups:

Here's how to do it -

Assume a pushup position, placing one hand on a medicine ball and one hand on the ground, and perform an explosive pushup. Rather than pausing and repeating at the top, continue through the explosive motion rolling the ball to the opposite hand and then performing a pushup . This will create instability and off load one side of your body with each pushup. This more effectively engages your abdominal muscles, the stabilizer muscles within your shoulder, and the explosive movements focus on the fast twitch muscle fibers within your chest, triceps, upper back, and abdominals.