I'm not too good with names of these moves, but I can describe some of the ab workouts my personal trainer has me do.
1) On an incline bench (like this:
http://t3.gstatic.com/images?q=tbn:ANd9GcT5lYLD_1D99iSCpFvQ6qwOc5jUf3VK4un89CoHU2_5XRhmbseEOQ ), legs between the leg pads at top, your head downhill on the bottom. Do sit ups as someone throws a medicine ball for you to catch over your head as you go down, throw it back to them as you come up.
2) Sit in an upright sit up position, lean slightly back and pick your feet up off the ground. Holding a medicine ball, touch the floor on each sit of your body, keep alternating:
http://t2.gstatic.com/images?q=tbn:ANd9GcS66zOOL1LkTqbryUcWdvH6Icp7_RtwtnIIG7gSVOwlqYVWbYQsiw
3) Back onto the same incline bench as in #1, lay with your head at the top, arms over your head holding onto that black handle. Lift your legs up like a reverse crunch, then continue to move up so that your lower back comes off the bench as well, letting your knees touch your elbows.
4) With some plates on a barbell (doesn't matter how much or how little), place the bar on the floor and sit on your knees behind it. With your back straight, almost like in a pushup position, roll the bar forward, stop, then roll back.
A six-pack will show when you get to a very low body fat %. Obviously, the more you work your abs, the sooner they will show, but no matter what, you need the low BF%. The 4 exercises I just mentioned should hit your abs pretty damn hard, if done right with enough reps. I'm sure you can get some others to chime in, as well (Fuglydude comes to mind...)