Use softwoods, such as pine or fir, rather than hardwoods in your fireplace. Softwoods burn more quickly than hardwoods but produce less heat energy.
Limit the amount of airflow allowed through your fireplace. The more airflow a fireplace receives, the hotter it will burn. By using a fireplace insert with glass doors, you can limit the amount of airflow allowed into the fireplace, which will make the logs burn more slowly and less hot.
Stack logs closely together in the fireplace to make them burn less hot. Logs that are stacked closely together limit the airflow around each log, which will cause them to burn slower and at a cooler temperature.
Partially close the damper once the fire has been burning for 30 minutes. Doing so will further limit the airflow of the fireplace and cause the logs to burn less hot.