Once you've mastered holding a plank for minute, you might want to try taking things up a notch. I found this to be a challenging new way to engage my core and keep my ab workout from getting stale. It's called a lateral plank hop and it works the transverse abdominis, obliques, and your heart.
To do the lateral plank hop, get into push-up position on an exercise mat, with your wrists directly underneath your shoulders. Hop both feet first to the left and then to the right, being sure to keep your core engages throughout, that means keeping your navel sucked in to your spine. Be sure to maintain a straight line with your body. Continue this pattern of hopping your feet side to side throughout the entire set. Try to go for 10 on each side.
Do 3 sets if you can, resting 30 seconds between each set. Let me know what you think.