I’ve got two main tips that I use to get more battery life out of my iPhone on long day trips away from a power socket.
Low battery mode
This is a great feature even when you have a fully charged battery. Head to settings, battery and toggle on Low Power Mode.
My next tip is a bit more drastic but works wonders.
Enable display accommodations
By enable black and white mode you can stretch your iPhone battery even further.
Head over to settings, general, accessibility, display accommodations, colour filters. Turn this toggle on and use greyscale.