It's honestly just practice. Figure out how to aim your hits. Almost every person has a different way of aiming their hits, they all work great, but find the one you like the most.
For speed, remember one thing, there's nothing wrong with grabbing. It can increase your speed a lot. Just figure out different ways to push yourself towards uke. If you grab, it's a simple matter of pulling uke towards you, while bringing your leg(assuming your kicking) towards him. Aimed properly and if you extend your leg at the right time you can achieve a large amount of damage. Check out shook's boomhit tutorial. There's quite a few helpful tips in there.
As for momentum, there's not much I can really say on this. I generally don't keep my momentum too much, I kind of try to build up a lot of it, very fast, but it's hard. I'd honestly say that keeping your momentum constant helps.
But yeah check out shook's boomhit tutorial and the how to make a madman tutorial as well. There's loads of helpful tips in both that can help you a lot.