With regards to clenching/unclenching the fist:
I don't believe that it actually makes a difference to the speed of your punch unless you're really trying to crush your own palm. Personally I clench my fist mid flight simply because I don't like to stand there clenching my fists all day, so prepunch they are lose.
I was once told by a very good boxer that he left his hands unclenched on his jabs before impact for the extra little bit of reach he could get. He said with gloves on it didn't really matter if he made contact with his fist unlenched, though he clenched it if contact was guaranteed for better power transfer. Basically a good way to score cheap points in amateur boxing.
If you wish to develop punching speed the only real advise I have is to get out on the heavy bag every day. Try doing three one minute rounds of just pounding the bag with straights as quickly as you possibly can, then rest for 15 seconds. If you have trouble with one minute rounds go for 30 seconds, key thing is not to overdo it so that you can keep getting out there and doing it every day (or 6 days a week). This is just a drill I became fond of, any punching drills will increase your speed, there is no secret or trick to it as far as I know.