Two very useful melee cancel tricks that improved my game game tenfold:
B - X - B: Simply hit B, then X, then B. You'll perform a double melee, usually killing your opponent. It takes timing and lots of practice. Try figuring it out on a wall, when you hear the melee hit the wall, hit x and b very fast.
B - X - R: Melee an opponent, and quickly hit X and R at the same time. Usually an instant kill if you're aiming at the head. I've been called a cheater many, many times for using this one. It takes lots of practice, and is very, very, very, hard to do.