With your client prone have them interlace their fingers behind their neck. Lift their elbows slowly toward the ceiling only as far as is comfortable. Have your client press their elbows to the floor using only 10% of their strength for 10 seconds. Then have them lift their elbows toward the ceiling and you assist them with the stretch at the end. Be sure to stay within your clients comfort range.
When the client lifts their elbows toward the ceiling they are engaging the opposing muscles (posterior deltoid, rhomboids). This utilizes the principle of reciprocal inhibition, and allows the pectoralis major to relax more.
I would like to thank Wes Barnett for this variation on the Pectoralis Major facilitated stretch. This variation allows for stretching lower and upper fibers of the pectoralis major.
March 21, 2016 - The video on this page was replaced with a better version.
Pectoralis Major PNF Stretch