Who have you paired with?
At one of our regular retrospectives a few iterations back, the team decided that we had a bad habit of not rotating our pairs enough. More specifically, we tended to pair with the same few people for all of our tasks. That meant we could improve the sharing our knowledge to other team member.
Perryn Fowler was charged with trying to rectify the situation, and he drew this chart up on a whiteboard shortly afterwards:
The idea here is that you put a mark in the square that intersects your initials and your pairs. We didn't enforce any rules other than that. Instead, we just let people come to the realisation that they were filling in some squares a lot, and others not so much.