I've found that when I'm in the backseat we can be most productive as a pair if I spend my time trying to keep the driver in flow. When I first tried pairing the programmer control freak in me made this very difficult. I was effectively trying to write code by commanding someone else's hands to do what I was thinking.

By focusing on keeping the driver in flow you begin to appreciate when it's better to shut up and let them get on with writing the code, accepting that people have different coding styles is important here. You can then use your attention to either help out when they get stuck or maybe to tell them what the next test should be.