Go to the phone settings, and turn off all features not in use. This includes, but is not limited to, the Wi-Fi, the GPS navigation application and the Bluetooth option.
If a Wi-Fi connections is available, leave the Wi-Fi on for Internet use as opposed to using the phone's data plan for such. Data usage drains the battery faster than using Wi-Fi does.
Go to the phone settings, and adjust the brightness to a lower setting. Also adjust the phone's timeout setting to shut the screen off after 15 seconds of user inactivity.
Go to the phone settings, and disable push notifications. Also disable push notifications and data-fetching within individual applications such as email and social media.
When no signal is available, go to the phone settings and place the phone into airplane mode. The phone stops searching for a signal, but Wi-Fi access is still available.
Free apps are ad-supported, and research shows the ads drain a cell phone battery faster. Purchase apps to avoid the ads.