I would like to introduce to you all my go-to ab workout in pretty much all situations: the bicycle crunch. While ab exercises are likely one of the fitness topics upon which you do not need more instruction, I wanted to highlight how to do this crunch, and how to do it effectively, because it's often one that I see people performing incorrectly. See steps below the pictures for more instructions!
1) Start with your hands behind your head, back flat on the ground, and extended left leg as close to the ground as you can get it without arching your back. Rotate your left arm to your right leg.
3) As you rotate your right elbow to your left knee, make sure to move your legs slowly and deliberately. Your legs should be tensed and strong the whole time. You do NOT want to flail them quickly - slow bicycle crunches are much, much more effective than quick rotations.
4) As your elbow meets your knee, make sure that your right leg is extended strongly and as close to the ground as you can comfortably hold it. Also, try to get as much of your upper back as you can off the mat and towards your knee.
5) Try to keep your elbows pointed away from your head, rather than pointing your elbows forward and wrapping your arms close to your head. This will be better for your neck.