Core Essentials - The Plank

When working on your core, one of the best and simplist moves I can think of is the plank.  It requires no equipment at all,  just a floor mat for comfort if desired.  It alone can be an excellent core exercise, and once perfect form is achieved, with a few adjustments the difficulty level can be increased for a more thorough core workout.  I've attached an illustation of this move that everybody should be practicing.  Remember - the most common error in form I see people making is lifting their glutes up in the air.  Keep them tight, low and level.