Theme should be as dark as possible.
Brightness set to low, unless you love the dark, in which case set it to auto.
If you rely on cellular data, use 2G on standby, 3G when in use. LTE only if you really need it.
If you rely on Wi-Fi, then stay within range and keep Wi-Fi on. If you want to save battery, just turn off every setting in settings>Wi-Fi>advanced
Switch off Location if not needed.
As a last resort, just keep Battery Saver on all the time, at the expense of Background tasks.