There is nothing worse than watching a stage slap or punch and seeing that the actors are no where near close to hitting each other. The first time I did a stage slap (1st year of college, don't you know) I didn't even know there was a way to fake it. I just slapped my acting partner full-on and she slapped me full-on back. The director kept telling us he was amazed at the slaps and how great they were so we just kept slapping each other harder and harder. My right cheek looked like a chipmunk with a permanent hand print.
Anyway, loved this article about the process of learning to fight on stage without killing yourself or others. The show is at Second Stage and is all about pro-wrestling (perhaps the ultimate in stage combat??).