To get into a plank, put your elbows on the ground, and position them directly beneath your shoulders. Relax your shoulders and neck, pull your belly button in slightly and then straighten your legs behind you. Make sure your back remains in a straight, neutral position—if your back gets tired first, your form is off.
Beginners should hold for five to 10 seconds. Work up to longer holds—45 seconds to a minute—as you get stronger.