The side plank is the most basic isometric lateral trunk stability exercise.
 
With your body in a straight line lying on your side, support yourself on your bottom elbow and the outside of your bottom foot.
 
Keep the entire trunk, glutes and quads tight to ensure you’re in a straight line and not rotated. Also be sure you don’t allow the hips to sag toward the floor.
 
Difficulty can be increased by adding weight to side of the hip. Place a dumbbell, plate or similar on the hip and stabilize with your free hand.
 
Notes
There will be a tendency to keep the hips partially bent and rotate the chest forward slightly—keeping the full body under tension will avoid this and make the exercise more effective.
 
Purpose
The plank is a simple but effective exercise to strengthen the ability of the abs to maintain the proper relationship of the pelvis and spine, in particular preventing hyperextension of the lumbar spine.  
 
Programming
The plank should generally be held for 20-60 seconds per set, but shorter (8-15 seconds) weighted planks are also effective. Generally more than 60 seconds is unnecessary and unhelpful—add weight to reduce time.