I can't say that I have six-pack abs - I still have more weight to lose, but I do feel like my 'core' is really strong. What was the best for me was the discipline that it helped with - that I had committed to myself that I would do it, and there were a lot of days when I was tired or had other stuff I would rater do, but I felt like once I broke the string, I wouldn't go back.
I didn't do the same thing every day - on the nights I lift I do a circuit at the end - not always in this oirder, but - using some contraption that I don't know the name of, I do some things that involve legs - pulling them up straight legged for 10 reps, then bringing them up with knees bent for 10, then 'running' in place for as long as I can stand it; then 20 reps on a 'chair' with weights, then 20 on a decline bench - sometimes 20 on a ball-then repeat.
Other nights I do 'floor abs - 11 or 12 different moves to try to hit different parts of the muscle. Sometimes I followed a set of routines on 'Fit TV'. And then one day a week I usually take a Pilates class. Varierty was a key for me.
I missed two days with a sprained ankle, one when I hurt my arm, and one was I was ill....so I felt that I had met the goal. I know a lot of people don't approve of working the same muscle multiple days in a row but I felt like this was good for me.