Well in my opinion with almost every punch you need to put a surtain ammount of weight into it, put too much and your opponent will bash you in the head and down you go, put too little and your punches wont hit as hard as they could.

I work on the jab, cross, hook technique and it doesn't fail me. Throw your jab, should be nice and strait with your arm almost directly over your leg, then you throw your cross, trying hard to keep it in the same path as your jab, but as your throwing your cross if you descide to put a hook in there make sure your close enough to hit them with a nice 90 degree elbow and make sure you keep it as tight as possible, it's scientificaly proven that a tighter turning circle creates faster movement, faster movement means a harder, more effective punch. At the end of your hook your ellbow should be almost directly over your knee, I find the secret to be that once youre arm is in its 90 degree position dont move it untill the punch is complete and to swing as soon as you finish your cross, once your hook is complete you should be in the right possistion to throw a jab strait away.

As for exersizes, I would maybe try throwing jab, cross, hook combos at a bag, don't worry about hitting the bag hard at first, because if you are throwing your fist wrong and hard you are gunna mess your shoulder up pretty bad, just worry about getting them nice and strait every time, then you can worry about spead, power and efficiancy. =)

Peace out, Luke.

Sorry if I missed anything important =S, if I have please feel free to correct me.