There was a thread asking the exact same question a couple months ago. I think almost everybody encounters this unless they tried to overcome it like someone said in the reply to the last thread.

I can't remember who it was, but they said not to think of "throwing" punches, but rather, imagine your arms leaving your side or in their poised position, then don't think of them travelling through the air, just make them immediately connect with your target, like some kind of dragonball z style stuff

I found this effective Try it

Hope it works =]
Jim.