There shouldn't really be any reason for your smart phone to be overheating, most smart phones are designed to have enough heat transfer in order to prevent that. However, that being said, usaully when you're charging the battery and using your phone at the same time is when it's going to be the hottest. So you could decide to plug your phone in to charge and then leave it in the sleep state so that it's using as little processing power as possible and then less heat output as a result.
Also try to avoid leaving your phone in your car, as the temperatures in your car can become 140F / 60C and above if the car is closed in a sunny area for over an hour.
