Mirror drill for pursuit and containment. http://www.footballdrills.com/linebackers/runstopping/mirror-drill.html. Make sure the linebacker runs laterally with the runningback and does not go toward the back until the back makes the cut.

I used to think it was just about pursuit. But some other things I would also look at are - Linebacker's reading the play. They need to read the play side back. If the Back is gong lateral then the LB should think" sweep. If he goes toward the line then more then likely off tackle. The other thing to look at is if the Lb is getting caught up in a Block.