to knife from behind make sure you're on his left hand side moreso than his right, or else it will only do 55-65 damage
you can actually 2x left click and 1x right click faster than 2x right click if you do it right. it will do 15-20-65 damage and will be faster than 2x right clicking by enough that you will get the kill. it takes a lot of practice and some luck since you need to be completely up against the guy at all times for it to really work out. but it isn't that hard to do since when you left click them they will slow down as if they were shot, allowing you to catch up to them.
just hope you don't actually do 15-20-55...cause then you'll still die :S
also if you're chasing somebody you can left click if you're close enough to tag em once in order to make them slow down so you can catch up enough to right click them for the added damage and kill. just do not continually left click while running, that slows you down.
with knife fighting you'll see people dodge in and out, its because you can draw them in so you run somewhat close to them and run away, they'll run forward and slash but if you timed it just right you'd have backed off enough that they cannot hit you, but you can run forward and hit them.
also since our servers do not have block on, you can run completely through them, turn around 180 degrees and knife real fast and tag them in the back giving you the kill!