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Switch off Wi-Fi, Bluetooth when you're out and about
If you are not near or not using a Wi-Fi hotspot, or sending items to other devices using Bluetooth, these can (and should) be turned off.
Under the swipe-up Control Center, tap the Wi-Fi button (second in from the left) off, and the Bluetooth button (middle) also off.
Enable "airplane mode" when you're... on a plane
Again, your iPhone's battery life may decrease far quicker because it is constantly searching and "pinging" for a higher-quality cell site with better signal, especially when it's not there. If you're on a plane or on the subway with no connectivity, you may as well squeeze as much life out of your battery as possible by switching off all radios with a single button.
Under the swipe-up Control Center, tap the Airplane Mode button (first on the left). You can tap it again to switch all radios back on again.
Disable 4G (and LTE) connectivity
While 4G is faster than 3G cellular connectivity, it also uses a lot more battery power. Long-term evolution (LTE) uses an entirely different hardware radio and often drains the battery even quicker. When not using high-bandwidth applications, streaming videos, or tethered to a laptop or tablet, you can disable 4G and LTE connectivity altogether. You'll receive slower download speeds (albeit still rather fast by modern standards) but it will also help conserve your iPhone's battery throughout the day.
Go to Settings > Cellular, then switch off the Enable 4G (or Enable LTE on newer models) button.