This post came through from the Sports Injury Bulletin. It discusses the plank exercise and the pros and cons of it.
I tend to use the plank in sessions I design as it is a relatively easy exercise to explain and a powerful core stability tool. The post includes detailed instructions as to how the plank should be performed and 4 possible cheats that are often used. I have seen all of these cheats used in the past.
The comments about keeping the pelvis in a neutral position and doing the exercise on a slippy floor with the feet on a slippy cloth are both excellent tips.