Well, think about it. Standing straight up, you can stretch your arms straight up and back, to the side and back (as if clapping your hands behind you),also clapping about your hand can provide a shoulder/lat stretch. You can cross your arms in front of you can grab the alternate shoulder (like a self hug) and one of primary muscles involed in punching is the pectoral, or chest muscle. I like to take a staff and hold it out so that it is vertical (from the floor to my hand) and my arm is its natural length extended horizontal from my shoulder joint. Then without moving the staff or my arm, I rotate my waist, which creates a mild stretch in my pecs. There are many varying angles of which to stretch a muscle, so investigate, naturally, the different angles which stretch the following basic muscles that aid in punching power:
If you have any further questions, feel free to email me and I will help as soon as I can.