Start with your arm from the outside or at your waist with a clenched fist, thrust up from your hips and feet, possibly jumping a little bit into it. Focus on the last couple of inches making solid contact like in boxing and make sure on a bag or some hard object that can sustain the punch that it's effective in those last couple of inches. That short distance of contact is what causes the maximum power that a punch is all about. Also it helps to lean over as your punching even with your head coming forward. You can be countered but if you land fast enough the fight is going to be stopped.