Medicine Ball Tosses - First of all, you obviously need a medicine ball, and not all gyms carry them. No worries though, you can find them at most sporting goods stores. An 8-12 lb. ball is ideal, and you'll also need a partner. Now, suck in that abdomen and perform the tilt so your back is in a nice, neutral position. You don't want to round out your back so you're almost bent over, so keep a straight alignment. Then, using the power of your abs (not your arms!), rotate your trunk forcefully and throw the ball to your partner. You should also throw it so your partner has to reach to the side and catch it at the same time, rotating his/her body with the catch. This will challenge the abs in an eccentric (negative) fashion. And now it's your turn to catch it the same way. Try three sets of ten tosses each.
The Chop - You will utilize pulleys for this awesome maneuver. First make sure you place a pulley attachment above the level of your head, and stand so one side (let's start with the right side) of your body is facing the weight stack. Now, reach your left arm across your body and your face (like you're sniffing your arm pit) and grab the attachment. Also grab it with your right hand. Of course you now need to suck in your gut and perform the tilt, and then use your trunk rotators to turn your entire upper body and torso left (don't use your arms). Crunch hard with those obliques! Perform three sets of 15 reps for each side.
Shoulder Presses with Sidebend - These aren't just for your delts anymore. These can be done in the standing position using the pelvic tilt and drawing-in maneuver, but I also like doing them seated with a back support. Now, I don't actually use the support except to use it as a guide to maintain my back alignment. Basically I make sure my low back stays in light contact with the pad at all times. If you let your back arch and come away from the pad, your pelvis just rolled forward and took your abs with it, right out of the equation. The twist? Perform alternate raises (one arm at a time) while performing a sidebend. In other words, as you raise the dumbbell in your right hand, you perform an upper body sidebend to the left. Try three sets of 8-12 reps for each side.
Scissors - Lie on your back, maintain a firm pelvic tilt and drawn-in abdomen, and (slowly!) kick your legs out alternately. This exercise is performed in the gym all the time, but rarely is it ever performed with even a passing regard for pelvic position. Also, most people do it so fast I often think they might take flight. So slow it down and keep your low back glued to the floor. Perform 15-25 reps, 3 sets.
Planks - Think you can perform a pelvic tilt against gravity? Try lying on your stomach and than raising yourself on your elbows and toes. You'll notice that your low back likes to sag in this position. You won't work your abs effectively unless you now perform a pelvic tilt, which is the next step. Suck in your abdomen and notice your back flatten (straighten). Do this next to a mirror to insure your entire torso is a straight line, and hold it for 10 seconds. If this isn't challenging, raise one leg a few inches off the floor and hold it 10 seconds, alternating legs. Try three sets on each leg.