Here is a nice progression for the plank exercise. Like always, spend time mastering the first exercise before moving on to the next. It's about what you CAN do, not what you CAN'T do.
1. Kneeling Plank - This is a good entry level plank exercise for those who not yet have a strength base.
2. Plank - This is where most athletes will start.
3. Feet Elevated - This is a good progression from a regular plank. A 12" to 14" box works best.
4. Marching - Anytime you take a limb (arm or leg) off of the ground during a plank, it now becomes an anti-rotation exercise. Make sure you don't bring the leg to high. You should only move from the hip - not the spine.