In today's world it is difficult to get around without a mobile phone. Taking that into account, if you are going to use one, it makes sense to think about the various aspects of frequency emissions from the phone and how they can effect you.
Rfsafe.com has useful information on how mobile phone frequencies are harmful to people. This and other sites also contain information about the SAR(Specific Absorbtion Rate) of each mobile phone. This is a measurement of mobile phone frequency radiation and how much it impacts the human body and head. Some phones have higher SAR ratings than others. It is a logical approach to take this into consideration when buying a phone.
But I have also found there is an even more important factor in deciding what type of phone you are going to purchase. This is the construction type and materials of the phone. You want to avoid metal and aluminium construction as these materials are conductors. So even if the phone has a low SAR, it will conduct that frequency directly to you. Generally the phone front is made of glass. It is advisable to go for a plastic type construction of the rest of the phone as this is an insulating material and will greatly reduce the frequency emission from the phone.
That would be my number 1 tip in looking at a phone, is the construction type - if it has a plastic construction then it is far safer for you in my opinion.
My second tip would be in relation to how you take calls. I would recommend using the speaker phone function on all calls that you can.
You simply hold the phone out in front of you, and not next to your head. This increases the distance between you and the phone while on a call, which is when the frequency will be at its highest.
I would not bother with using headphones for calls as this is just channelling the frequency directly into your ears and to your head. Using headphones on a mobile is not mitigating the problem; it may even be making it worse for you. Of course, if you can try to reduce the number of calls you need to make on your mobile this will help also.