Ive recently joined the gym and I have a ab workout, after just 1 week im already seeing a large difference. I do three sets of this:
30 crunches
20 butterfly kicks
30 heel touches
30 cross crunches (15 each side)
30 crunch cycles*
1 min plank
30 supermen**
* you hold crunch and move your legs in a cycle motion
** lie on your front, with your hands behind your head and lift your head and feet off the ground, this helps stretch out the muscles ready for your next set
hope this helps