Cheaper: Pop the SIM card out and wrap the phone in several layers of foil. Put the SIM card in a little baggie and keep it (and the phone) with you. If you break down or have an emergency, you can still fire up your phone and call for help, but as far as I know, if the phone is off and the subscriber identity module is removed, it's dead in the water.

EDIT: I have since learned that this is not true. The phone still gets tracked by the IMEI.

Cheaper: Pop the SIM card out and wrap the phone in several layers of foil. Put the SIM card in a little baggie and keep it (and the phone) with you. If you break down or have an emergency, you can still fire up your phone and call for help, but as far as I know, if the phone is off and the subscriber identity module is removed, it's dead in the water.