Kneebar is pretty much my second favorite sub, after the almight omoplata. Favorite setups:
Half guard (no brainer ... I like to set it up by faking the choke and then spinning).
Side control (particularly when the knee to prevent mount is up).
Turtle (loooove this one).
Open guard (any time you get an underhook, the kneebar is there).
Omoplata finish (if he's sprawled out ... switch to the kneebar).
Scarfhold (amazing how easy this one is. You literally just step over, take the knee ... and done. A lot of times your opponent will foolishly think he is getting some sort of halfguard by jumping into the kneebar!)