Have you ever watched an episode of Dragonball Z? If you haven't heres a quick description...guys with goofy color changing hair that scream and look constipated with veins popping out in their foreheads that are so fast they can beat the crap out of you before you even blink. There has been a few times I've sparred with (never fought thank god) guys like this, guys with ridiculous hand speed that can hit you before you even react.
These speed demons are generally born with an innate gift and physiology for being really fast but anybody can increase their hand speed and quite substantially I might add. There have been a few experiments with hand speed and there have been results yielding as much as a 50% increase in hand speed, if I could punch 6 times in the same amount of time it took me to punch 4 times right now I would be pretty psyched. I've tried several ways myself and I'll share the one that gave me the best results.
I used resistance bands because its a bit easier on the joints and tendons so you can train more. Punching with dumbbells usually made my bicep and forearm tendons a bit sore for the next few days because of the pulling back, bands allow you to train more often. Plus I have a cool setup for them :-) I'll share that one of these days.
1) Pick a 5 punch combo (we'll go with 1-1-2-3-2)
2) Ladder up to doing the full combo
3) Take a very brief rest between full reps (working up to the full combo)
4) Repeat 100 times
quick step in - jab - duck and step right
quick step in - jab jab - duck and step right
quick step in - jab jab cross/body - duck and step right
quick step in - jab jab cross/body left hook - duck and step right
quick step in - jab jab cross/body left hook cross - duck and step right
Rest 5-20 seconds and repeat 100 times (for a total of 1500 resisted punches...not too shabby!)
Remember to rest enough so that you feel like you can move at maximum speed or near max speed because your not working on endurance here, getting a nice burn has no place in speed training. However some additional and very sport specific endurance is gained from doing 100 reps of this and that is why I did it this way, I may as well be able to punch fast for a long time right? I may have not gained as much raw speed as possible but the speed I gained was always available to me when I needed it and not just in the first 30 seconds of a round. Working in footwork is always a good idea as well, you rarely punch without footwork and you rarely land without it either so being able to punch fast stationary is nearly useless unless you just want to be able to beat up a heavybag.
Overall I believe this method of training hand speed is very good and it has some additional benefits that are very relevant to boxing performance or whatever other combat sport you are involved in.