The only sure way is to stop using a cell phone. Most people however would prefer to play safe by simply reducing their exposure to radio frequency radiation. Bluetooth devices emit only 10% of the radiation of a cell phone. A hands-free set enables the user to keep the phone away from the head but usually leaves the phone near some other part of the body. It is safe to say that in the event that radio frequency radiation causes damage, you would like to protect other parts of your body. Therefore the only way to reduce your risk is to reduce the radio frequency radiation your body absorbs. SAR Shield reduces radio frequency radiation by up to 89%.
The energy of electromagnetic radiation is determined by its frequency; ionizing radiation is high frequency, and therefore high energy, whereas non-ionizing radiation is low frequency, and therefore low energy. The NCI fact sheet Electromagnetic Fields and Cancer lists sources of radiofrequency radiation. More information about ionizing radiation can be found on the Radiation page.