Exercise! - Core Muscle Strength Training

Looking for advice on the best routine and schedule for core strength training.

Is an exercise ball best? Also can I work my core muscles daily or should it be no more than 3 sessions per week? :?:


Thanks!

My trainer is having me work out my core daily with same exercises. I've had doubts that this is good for me, but my body is responding, and core is getting stronger.

I do a variety of floor and ball activities.
Floor and ball crunchs, inch worms, one legged v-ups on ball, cross leg crunches and ball obliques.

I recently found Reverse Incline Bench Sit Up and Keeling Heel Touch that I am curious about.