You may want to offer a few free sessions geared toward exercises that will help them become better climbers and improve their overall strength and conditioning for climbing. Give them some tips on how strength training can benefit their overall health (ex. more lean muscle mass keeps the metabolism burning, proper strength training techniques prevent injury, how proper stretching after a workout can decrease muscle soreness and aide in recovery, etc). People love good free advise. Take a personal interest in each individual. When people perceive your personal interest in their wellbeing they are more likely to trust you and sign up for some sessions.