Steady hold factors for prone position:
Support hand open, cradling the rifle
Sling smooth across the back of the hand, high in the bicep, snug, not tight
Support elbow directly underneath the rifle
Support side leg in line with spine
Trigger side leg, high, knee a 90 degree angle
Trigger side elbow planted
Firm handshake grip pulling the buttstock smartly into the shoulder
Exaggerated C-shaped trigger finger
Turkey neck, cheek weld.

The Ambidextral Gunfighter
http://www.ambgun.com