Buy a Headset with Mic:
First and foremost tip i would suggest is to Buy a Headset with Mic and keep the phone away from your brain. Wireless headsets also a good option when it comes to your comfort. Nowadays, Bluetooth headsets are available which will reduce mobile phone radiation.
Do not buy a cheap mobile phones because cheap phone companies do not take care in manufacturing low radiation mobile phones, if you take Apple or any big companies they really take care of the users also and they produce mobile phones which are really secure and these mobile phones produce very low radiation.
Keep the Mobile phone at a distance : If you do not have a headset , just make sure you do not keep the mobile phones near your body. Keep the mobile phone at a audible distance and talk. Whenever possible if you are in a private room you can switch over to Hands free.
Reduce Talking with Mobile Phones : Avoid friends who waste your time or talk for long time, Just send SMS and chat with them through SMS rather than talking with them.
Do not talk when the signal is weak : Do not talk when you see the Connection bar low , because when it is low technologically speaking it will emit more radiation to complete the call successfully, do not speak on the mobile phone while the signal is weak.
Remove all the Protection: You may have bought a costlier smart phone , you can put cover for the phone, remember these covers will stop the signal to the phone, this might force your phone to reach for the signal which ultimately results in higher radiation, so remove all the covers and talk freely.