That's kind of the easy way of training core. To step it up a notch, if you want to do it the cheap way:
Find a surface where you have a hardwood floor. Get two paper plates and place them under the balls of your feet. Come to a full push up position, keeping your hands flat on the floor with your fingers fully splayed, shoulders flat (don't let your shoulders wing), and abs zipped. Then, while keeping your hands exactly where they are, slide your feet towards your hands as you hike your butt up towards the ceiling (I call it a "pike" or an "inverse V with the body"). Then, slowly slide yourself back to a push up position. Do not let your midsection sag or your stomach pop out at any time.
How about if you do that for say.... 5 minutes.
Then, immediately take it to mountain climbers. While still in the same push up position, alternate sliding the feet forwards one at a time, pulling the knee into the chest as you drive the leg forwards. At the same time, the other leg will extend back as far as possible. Again, keep the midsection from sagging, the shoulders from winging, and the abs drawn in and zipped up.
How about if you do that for another three minutes.
Finally, come to a standing position. Keeping the legs as straight as possible, bend forwards and get your hands to the floor (bend the knees if you don't have the flexibility in the hamstrongs, but only as much as possible to get your hands to the floor). While keeping your feet exactly where they are, walk the hands forward until you're back in a full push up position. Do 1 push up, and then flip to a side plank on the right. Take it back to a push up position, and flip to a side plank on the left. Take it back to a push up and give yourself 1 more push up, then walk your hands back until you are in a standing position, bent over. As much as possible, straighten your legs and roll your torso up to an upright, standing position.
This should be done as fast as possible. Let's say you'll do this 20 times.
Do this routine 3 times a week- you will see an awesome adaptation of your core muscles.
This is a unique and fun way to work the core muscles- I don't like to put my clients through the same old crunches and stuff.
koffee