LOW POWER MODE
The most obvious and simple solution is to enact Low Power Mode but it's not something you want to be using all the time.
Your phone will automatically suggest you use this mode when your battery drops below 20 per cent but you can switch it on in the Settings under the Battery category to conserve some mobile juice when needed.
The mode turns down the display brightness, reduces the time for auto-lock to kick in and disables various features like background app refresh and automatic downloads.
TURN OFF SIRI EAVESDROPPING
If you're not that keen on Siri being at your beck and call every second, you can turn stop the digital assistant from constantly listening to you. You can still use Siri by holding the home button, but she (or he) just won't be listening out for you to say "Hey Siri" to engage.
You can turn the feature off by going to Settings, then Siri & Search, then turn off Listen for "Hey Siri".
TURN OFF ASSISTIVETOUCH
This feature is for people who have difficulty touching the screen and is designed to assist with your finger dexterity when doing gestures like pinch or multi-finger swipe on the touchscreen.
If you happen to have it turned on, it will contribute to draining your phone's battery so to switch it off, go to Settings > General > Accessibility > AssistiveTouch.
TURN OFF EMAIL FETCH
If you're not desperately waiting on an e-mail from your boss, you can stop your phone from constantly searching for new messages dropping into your inbox. Instead, it will only refresh to look for new emails when you open the app. You can also change it from automatically fetching data to doing so every 30 minutes of every hour.
To toggle with this feature go to Settings > Accounts & passwords > Fetch New Data.
TURN OFF BACKGROUND APPS
You can also stop your phone from working to refresh the data on all your apps as well.
Even if you're not worried about chewing the battery, this is a good thing to do for those trying to be less distracted by their smartphone by limiting which apps are able to refresh themselves in the background.
Go to Settings > General > Background App Refresh and there you can choose which apps you want to stop from constantly updating in the background.