One of the best core workout's I have ever had the pleasure of being tortured with (in a good way) was from an instructor of a general fitness class I went to for college. We would do it very class after our regular work out. I took the class twice a week and by the end of the semester I had a six pack, and visible obliques (the V-cut.)
The workout went like this:
1 set of 30 leg drops - lay on back with feet straight up, drop feet to 6in off floor and bring them back up.
1 set of 30 hip lifts - lay on back with feet up, lift hips straight into air, then back down.
1 set of 30 russian twists - I have also heard them be called "cherry pickers." you sit with chest up, reach to the right while rotating shoulders, then to the left.
1 set of 30 side crunches (each side) - lie on side with bottom hand on oblique and top hand on head. lift your toes and shoulders up towards each other, bending at your top oblique.
1 min side plank (each side) - Lie on your side resting on your bottom elbow, and feet together. Lift your hip off the floor and hold yourself there for 1 min.
1 set of 30 side raises (each side) - After the plank, drop your hip down and lift yourself back up into the plank position.
1 set of 30 side reaches (each side) - holding the plank position, take your top arm bring it in front of your body and reach under your bottom side, look and rotate under your bottom arm pit while remaining balanced.
2 sets of 30 regular crunches - if those are easy straighten your arms by your ears as a counter-weight to make it more difficult.
1 set of 30 super-mans - lie on stomach, arch up lifting your legs and upper body into the air. Hold and then relax - lowering yourself back down.
Finish it off with a 2 min regular plank (iso-hold)- go on elbows, similar to a pushup position. keep your torso level and hold it there.
Now obviously, this workout can be modified a bit until you can reach this level. Start by picking a few exercises and doing have as many reps as listed. Work your strength up and begin to add more reps and more exercises. 15-20 minutes a day of core strengthening exercises added to your regular exercises should be good way to begin to see results.