Thought i would give a little advice i was given from one of our clubs former players who plays against county and international players.
Number 1, gripping the ball, only have 3 points of contact on the ball, the more points of contact the slower the ball will release from your hand (i.e. dont rest fingers on the side).
Number 2, Run up, i go for a 8 and 8 run up totalling 16 once you hit the last 8 of your run up you accelerate to the crease, if you have a long run up and dont accelerate, its a pointless run up, wasting your energy.
Number 3, Core, your core is your key muscle when bowling, strengthening your core, obliques and lats will improve your bowling speed.
Number 4, off-season, buy a resistance band, practice bowling, keeps your muscle memory.
*i am not a Cricket Trainer, these are what have been advised to me. May not work for everyone.