Unless you need it at a particular time, turn off Bluetooth, GPS, Wi-Fi, mobile data, & syncing. Keeping these communications options running continuously will drain your battery power due to the constant polling of the network. I have set up profiles on my phone using Elixir (there are similar Apps available), that control my settings throughout the day. When I’m at my desk I turn off all the unnecessary communications settings under a “Conserve” profile, as I only need the phone to make & receive calls.
Turn down your phone’s brightness. While many smartphones have an “auto” setting, you might find it useful to disable this & manually keep the brightness down low, but to a level which is still readable. Be aware that setting the brightness down low may make it very difficult to read the phone if you move to a very sunny or bright area.