What are the techniques to create a realistic fight/combat scene without injuring the actors?
The fight is shoot in many small pieces,in movies you can see how camera always changes many positions during fight scenes.
The camera position faces so that the fist blocks the view of impact,actors keep the distance between them so actually there is no real contact between them,the sound and the impact effects are added later on.Each scene is reviewed to make sure that actor that gets hitted timed accordingly to the first actor that is hitting him so that it looks believable to a viewer.
Also actors can perform parts of the fight in slow motion to make the best timed hits and other complicated awesome moves,it is edited and fixed later on so it looks like real time.
In movies like matrix with longer and steady camera scenes the fight requires a lot of choreography from actors and from the cameraman and it includes a lot of extras and ropes.
The moves are all scripted, I believe; so every actor knows which hits are coming up next and it's not a surprise...unless it's improvised like a slap or something (in which case that is totally real).