Same technique as the mountain climbers, but with both feet thrusting in toward the body at the same time.
Be sure to keep your back straight, not arched, and resist up-and-down hip movement by contracting abdominal muscles.
Start out with a slow, controlled movement and work toward increasing speed without hip movement.
2 sets of 10 second controlled strides.
Over time work up to 30, 45, or 60 seconds.
Also progression can be made by increasing the number of reps over the time.