Attack infantry from the rear/sides with them, any unit that isn't peasant infantry/skirmishers will negate the charge anyway.
But it's fun to see people flying all over the place when you roll into a group of peasants with knights.
And only use them as a demotivational force, ergo when you're done with the charge, quickly get out and do it again at a weak spot.
Mass routs are great, my fav part is hunting the runners down with fast light/medium cavalry. Love them falling, like little ants.
Also, always have at least a General cav with the armies, or 1-2 light cavs to chase down runners. Thanks to the dumb AI movement when chasing runners, infantry won't really get close, even cav needs constant movement orders to kill something.