Backing off simply encourages them to hiss more. It means they don't want you to touch them, so they hiss. You back away, giving them the thought that hissing = you go away.
If they hiss or try and bite, leave your hand in front of them, but don't move it. Keep it there for a while, out of their reach, so they know hissing won't really work.
This is a work in progress with Monty.