It's all one muscle. There has long been a misconception that there are "upper" and "lower" abs, but in fact your abdominals are one long, flat sheet of muscle that runs from the bottom of the breastbone to the top of the pelvis. You work the whole muscle with virtually any abdominal exercise you do. Reverse crunches are no more effective for working the lower fibers of the abdominal wall than standard crunches. If you want to get more of those lower fibers working, concentrate on keeping your abs pulled in tight as you do each repetition; focus especially on keeping that "cummerbund" region below the belly button pulled inward.
Mix and match. Your muscles think it's a big yawn to do the same exercises week after week, so after a while they stop responding. That's why even though most ab exercise you do target the same muscle fibers, it's a good idea to change up your routine on a frequent basis. Throwing new and different moves at your abs keeps them in a constant state of adaptation, and ultimately that translates into better results.
Don't wait to exhale. Breathing is perhaps the most neglected yet most important aspect of ab training. By exhaling strongly through your mouth as you lift and inhaling through your mouth as you lower, you work deep muscle fibers that would not otherwise get into the act.
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