I usually train straight-set/ consecutive sets for 2 weeks, then superset for 2 weeks, and then circuit or HIIT for 2 weeks. Depending on whether or not I'm looking for strength or long lines. This is what a sample core workout looks like for me. I've trained this way for awhile and it works.
Warm-up: 1min. marching knee lifts
3x 12-20 hip lifts