I found this 15-minute ab exercise on YouTube the other day. Most workouts are recommended to last 15 to 20 minutes. I do this when I wake up in the morning and it really works. You can't flatten your abs just by doing ab exercises alone because you can't "spot" train, but these exercises help strengthen my abs and lower back. This workout, along with other exercise and a healthy lifestyle will really help keep a person in great shape.
The girl on the video is very informative on demonstrating how to do the ab movements. I highly recommend this video. It doesn't take long at all to do this once a day. You'll feel better every time. :)