I love circuit training if you haven't already guessed. I believe whole heartedly that circuit training is one of the most effective ways to design your workouts. Look at p90x....circuit training. Crossfit................circuit training. Bootcamps................circuit training. There are so many differnt versions of circuits that you can do. Jackie once again is my test subject for a miserable circuit. Here she has abreviated a circuit that would normally be done with 1 minute intervals. When performing intervals, have a high and a low. The high can be plyometrics and the low can be abdominal exercises. The high can also be resistance based exercises and the low can be aerobic exercises. More than anything just mix it up. Look at what Jackie is doing and give it a shot. She wanted to kill me after this.