Setting these up before you travel makes a big difference:
Quick settings
- Turn off automatic app updates: iOS "Settings → App Store → Mobile Data" / Android "Play Store → Settings → Network preferences"
- Turn off video autoplay in social apps — video is by far the biggest consumer
- Turn off photo and cloud auto-backup: set iCloud and Google Photos to sync on Wi-Fi only
- Enable low data mode: iOS "Cellular → Cellular Data Options → Data Mode → Low Data Mode" / Android "Network → Data Saver"
Before you leave
- Save maps offline: Google Maps lets you download the areas you'll visit
- Download music, video, and translation packs in advance
While you're there: do app updates and video streaming on hotel Wi-Fi, and stick to maps and messaging while out. Track your usage anytime in the app or account page.