Starting in a plank position, keeping abdominals very tense, lower down to one elbow, then the other, and then back to the palm of the hand one at a time. Hand, hand, forearm, forearm. Important: Make sure your shoulders are over your hands.
Try this alternating each way and you will find it is not only working your abs and upper back, but your arms as well. If that is too tough, start on one knee or both, but make sure the body is at a diagonal - not slumped and sagging.
And if that is too hard on your wrists, try holding the plank on your elbows, dropping one knee to the floor and then bring it back up and try the other leg. Alternate for 60 seconds.