I have tried various methods, such as knitting and stitching with resistive thread, applying carbon paint to jersey, mixing conductive fibers with stretchy fabric glue, stretching various conductive materials to see if this changes their conductive properties… and some of it worked, sometimes inconstantly or even incoherently or just wore out over time too quickly.
also see: knit stretch sensor, fabric stretch sensor
Leah Buechley gave a workshop at Mediamatic, introducing a lot of materials that can be used as stretch sensors.
>> http://www.imagesco.com/sensors/stretch-sensor.html (USA)
>> http://www.merlinrobotics.co.uk/merlinrobotics/merlin-stretch-sensors-c-33.html (UK)