Students tend to prefer to be the driver. Assign which student will start in each role to avoid pairs beginning with a difficult negotiation. Throughout class, ensure that students’ chairs are positioned so that they can both see the computer screen. If applicable, specify which computer the students should use to avoid a negotiation about this.
Example: "The partner on the left will be the navigator first."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-pair-programming