High frequency training builds abs the best. For me this means hitting them 4-6 times per week. Four sets per session is usually adequate. If pressed for time, and you can't feasibly hit a 4-6 day a week frequency, then do a two-exercise superset where the first exercise is predominantly lower-ab focused followed by an upper ab exercise. An example would be hanging leg raises immediately followed by rope crunches! ~ Berceste