Here is a very sensible waist routine that Rick Wayne used to beat Franco Columbu for best abdominals in the Mr. World contest.
Rick had an injured back and developed the following routine to work around his back injury. He used three exercises; cramp sit-ups, side bends and leg raises.
The cramp sit-ups were half sit-ups with great concentration and tensing. Vince Gironda taught a similar method. These were done for 10 to 15 reps as part of a tri-set.
Side bends were done withouit weights but with great concentration and tensing for 15 reps as part 2 of the tri-set.
Chinning bar leg raises were done as the third part of the tri-set for 15 reps.
Three tri-sets were performed with the idea of working up to five. Start with one tri-set, after four workouts move up to two tri-sets.
You will notice these are low rep exercises. Rick Wayne felt the waistline should be worked in the same fashion as any other muscle groups. He was not a fan of high rep waist work.