My favorite quick and effective ab workout are my bench abs. They only take a few minutes, but the burn last for a couple of days. It works your abs and obliques as well as testing your stamina.
The first step is getting a bench (a long, sturdy coffee table works just as well) and laying on it, facing up. Your upper back should be hanging off the back. If the bench doesn't have a place to secure your feet, ask a friend to sit on your lower legs. It's very important to have your legs secure, otherwise you can fall or strain your back while trying to remain balanced.
Extend your arms out and reach straight back, trying to reach the floor. Raise yourself back up again. It's like a reverse crunch. You can also alternate by extending backwards to your left and right as well in order to work out your obliques.
The entire time you should be concentrating on your abs for control as you extend up and down. If you're a beginner, all you need to do is two sets of 15 to feel your abs burn the next day. As you improve over time, you can increase your sets and add weights!
Be sure not to strain your back or neck! It's very important to exhibit control with your abs and back. If something does not feel right, adjust your position on the bench.Do you do bench abs? What's your favorite quick and effective ab workout?