The problem most people have is if they use their AR and pistol, and think they HAVE to knife (they really should try to reload unless they know they'd be shot without a doubt doing it), they just pull their knife and charge straight at the enemies. This is a very bad idea. What should happen is you (the knifer) should sprint at about a 45 degree angle towards them until either they are out of ammo, or you are at the point in the sprint where you can turn 135 (or so) degrees and be practically next to them. Most people just sprint directly towards people, hoping they can get close enough to attack. ARs have a very good chance of killing you if they have a straight shot at you, but targets traveling at an angle are much harder to track.
Now once you get close to the target (within about 5 feet) comes the attack. There are several ways you can do this. Generally on my first strike I will try to sprint-knife the person. This is done by sprinting directly at them and when they are just outside of range quickly stop sprinting-click-continue sprinting. THis move allows the knife to attack while you are traveling forward as fast as possible.
If your first pass misses, or you get a rubber knife, turn immediately back in their direction.[/i] This has to be done quickly, or your turn will be too wide and they'll have a decent shot at you. In fact, you should always turn around for another attack even before you know you've killed someone with the knife.
Basically you should just continue running around in tight circles and dodging around the guy, attacking when you see an opening, and sprinting out of his lane of fire whenever possible. Proning (not bunnyflopping) occasionally will help throw off his aim, but immediately press control after a prone, as their is a delay to press Z or space again. Remember, never try to occupy the same point in space for any longer than you need to.