Try this 12min. ab work-out I do...
3/4 Situps (3/4 so you don't get a chance to rest at the top)
Split Scisorres (on back, one knee to chest, other extended, both legs at slight angle towards extended leg, slowly switch, pelvic tilt)
Alternating situps (left elbow to right knee, then switch)
Flutter Kicks (must be in a pelvic tilt, very important)
Rockies (sitting, grab your knee, lean back 'till arms are staight, look behind you and touch the floor while leaning back, switch to other side and so on)
Bum ups (on your back, legs straight up in the air, know lift your butt off the ground, repeat)
Wide and alternating (on back, legs wide apart with knees bent, reach forward and grab the out side part of your knee with both hands, repeat on other side)
Leg extensions (on back knee to chest, extend legs untill parrelle to floor, the pull back, must be in pelvic tilt)
Obliques both sides (lay on your side, and basically do a sideways situp as far as you can)
Crunches
V-Sits
1 min. each, no breaks
Pelvic tilt: Lay on your back, feel the space under your back where your spine curves, make that space go away. Very important for the ones I listed include a pelvic tilt, without it you are waisting your time.