Pretty simple actually. Never lock your elbow, EVER. When you punch make sure you punch like a tiger, very quick, without locking your elbow, and very important, USE YOUR BODY!
When you punch, stand a bit sideways with your strong leg a bit behind your weak one, and make sure to not lock your knees, bend them a bit.
After all the preparations, you stand side ways and all, make sure your punching hand's shoulder moves a bit forward with your punch, so it will defend your open side when you punch. Keep the same stance you had before. And most importantly, don't be afraid of the pain, and you will punch twice as powerful.
Thank you mate.