a really really good way to improve your punching power by impressive amounts is to shot put.
get a dumbell. go outside into your garden. throw it as far as you can with the right hand 5 times, then change sidees. the act of hurling against resistance will really build up the same muscles used for straight punching.

but remember, power comes from the conversion of bodyweight behinf a punch, not the muscles in the upper body. improve your timing and co-ordination of the punch. thrust your hips into the shot and rotate your upper torso into it.