1— do not move your fists backwards before you start to move forwards, doing this is telling your opponent where the punch is coming from before it starts to come. keep your fists on guard and when you are going to hit just hit, forwards never backwards and dont hesitate.
2— yes, punch with the two biggest knuckles and try to make contact on bone with them.
3— just like @davidbetterman said, you need to pull back after contact. this is to do with physics and how energy transfers. when you punch your opponent, if your fist remains in contact with him, first it turns the punch in to more of a push, but more importantly, staying in contact with your opponent via the fist means some of the energy you delivered to his body is going to bounce back and be reabsorbed by your contacting fist and arm. pulling your fist away immediately after delivering the blow will trap this energy in his body and will indeed cause considerably more damage.
4— try to trick them, just before you smack em full on with your right fist, try pretending that you are about to punch them with your left but then change your mind and deliver a right. the confusion will give them less reaction time and you also have a better chance of catching them in a blind spot. i find fake left to the stomach followed by a real right to the side of the nose is very effective indeed.
5— dont start fights, only fight when you really have to and dont be the first to hit. why? is this just some goody goody advice? no… for starters you will avoid legal problems better if you dont start the fight. but it has strategical value too. nothing is going to damage your opponent more, than punching them with your bast shot just as they take a step towards to to punch you. not only do they get hit by all the energy of your punch, but they also take all the energy they put in to stepping forwards to punch you at the point of contact.
wait.. keep eye contact with his shoulders in view, and as he starts to punch move out of the way of the punch and punch them instead while you also step forwards.
i guarantee, if you hit someone like that the fight will be over instantly. they will just lose consciousness and you will be safe.
note: if you do hit someone like this, and they get up, run! ... run for your life because you just hit superman.