blk had asked for situp instructions, so here's my best attempt at explaining them. I just didn't want to throw another comment on an old post. I hope they're useful to people. :)
Normal crunches: flat on back, knees up, hands behind head (normal situp position). Concentrate on using just abs to bring your upper body up, and remember to exhale as you contract up, inhale as you go down. Also make sure to keep elbows behind your head, it really changes the difficulty I've discovered.
Reverse crunches: flat on back, legs up in the air, cross ankles but keep legs mostly straight. Contract the lower abs to bring your butt off the ground, but don't swing your legs, just use the abs. Go slower for more challenge, and again exhale as you contract and inhale as you let go.
Side crunches: lie on right side, with right leg slightly bent but mostly straight. Put left knee behind right knee on ground. Put arms behind head in standard situp position, with your right arm mostly on the ground and your left arm in the air. You then contract the sides of your abs, as if you're trying to connect your left hip with your left shoulder. It's kind of a side crunch with your head coming up a little on the front, and I go up about as much as it takes for my right elbow (which was flat on the ground with my arm) to be "supporting" my head. Also bring in your left hip and left shoulder when you do this one, again contracting just abs and not using leg muscles. Try it a few different ways, hopefully you'll figure out how to do it. It took me a while to get it right, and was mostly by feel. Also, exhale when going up and inhale coming down.
Back press: Lie in standard situp position, with arms extended and palms flat on ground near your hips. Press the small of your back to the floor, trying to push it through the floor. This will tip your hips up a bit. Then tighten your tummy as much as possible, continuing to press your back into the floor, and hold for as long as possible. Breathe shallowly so as not to expand the tummy.
I do ten each of the crunches, reverse crunches, and each side for side crunches, plus at least two back presses held to the count of 15. I do this every day except Sunday. :)