The proactive and spontaneous delivery of services for mobile users on the move can lead to the depletion of users' mental resources, affecting the normal processes of their physical activities. This is due to the competition for limited mental resources between the human-computer interactions required by services and the users' physical activities. To deal with this problem we propose a service selection method based on two theories from cognitive psychology. This mechanism assesses the degree of demand for mental resources by both the physical activities and the services. Additionally, a service binding and scheduling algorithm ensures less cognitively-taxing mobile service compositions.