Holding the standard plank for ten seconds or longer is useless according to Dr. Stuart McGill. Unfortunately, many people still try to do planks for time.
A better way to progress this core exercise? Make it harder. The cable plank press will help you take your core strength to the next level, and you'll even build some extra upper-body muscle.
I'm going to describe a little 6-week program that I call Tsunami Training (I'll get to why I call it that a little later). It's based, for the most part, on training methodologies that I picked up from Ian King and Charles Poliquin, with some razzle-dazzle thrown in by myself.