There's a number of tutorials available- if you get the "Ultimate Fights volume 2" they've got a how-to on there, just for example:
http://www.amazon.com/Ultimate-Fights-Vol-Al-Pacino/dp/B00005YXCK/ref=sr_1_7/104-1917885-3234304?ie=UTF8&s=dvd&qid=1184539549&sr=8-7
The above answers are all right in their own way... you can make contact with body parts, especially if they're padded under the costume, but actors can generally commit more to an attack (making it look better) if they know they aren't really going to hit the other guy, so I usually prefer the non-contact hits for all but close-ups.
It's all about having it look like the weapon (fist, foot, etc) and target (face, body, etc) are in the same place at the same time, which means one is in front of the other so they overlap. This is all about angles, from the director & dp's & choreographer's point of view.
For the actors, it's more about timing, since to get that correct angle the person getting hit has to react at the exact time it appears to the camera that there's been contact. Most common mistake is for them to react too early, then we see the gap between the fist and face, or it just doesn't look like there was any real force behind the impact.
But yeah, short answer is to find someone who knows what they're doing... although that can be hard to judge if you don't know yourself and if you don't have examples of their work to look at. Have them do a screen test for you if you have the time.