Create a Scratch project to play music to indicate that students should switch roles. If students are physically able, have them stand up and switch seats when they switch roles. If a student won’t relinquish the driver role, their partner will be standing up as they wait for them, which allows you to intervene. If students work in a group of three, have all students rotate seats every time to make it easier for them to track the rotation of roles.
Example: "When the music plays -- stand up and switch roles."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-pair-programming