If you're the kind of person who likes watching Netflix in the bus or listening to Spotify at the gym, take the habit of streaming your content offline instead of directly on your data. It may sound complicated but it really isn't, it basically means that you need to think about downloading your favorite songs and shows when you are on WiFi at home so that it's available for offline use later. Bonus: you'll even be able to enjoy your content in the metro without internet access!
2. Turn off WiFi Assist on iOS
This is of utmost importance if you are an iPhone user: do not ever enable the WiFi Assist (and I really mean never). Basically, WiFi Assist is a feature that allows your iPhone to stabilize your internet connection by using your data, and that is even when you are on WiFi. Unless you have a very faulty WiFi (and even then), this setting should remain off at all times. If you've been receiving bills for more than 10GB per month since you own an iPhone, that's probably why.
Settings > Cellular > WiFi Assist (all the way at the bottom) > Off
* While you're already there, you may also want to turn off the data for iCloud Drive.
3. Disable automatic downloads on data
iPhone: Settings > Apple ID (name/email) > iTunes & App Store > Use Cellular Data > Off
Android: Play Store > Menu (on the left in the search bar) > Settings > Auto-update apps > Auto-update apps over Wi-Fi only
4. Turn off background app refresh
iPhone: Settings > General > Background App Refresh > Background App Refresh > Off or manually turn off for each app
Android: Settings > Connections > Data Usage > Data Saver > On
5. Disable data for data-hungry apps
iOS: Settings > Cellular > Manually turn off for each desired app
Android : Settings > Connections > Data usage > Mobile data usage > Select desired app > Allow background data usage > Off
6. Disable video autoplay for Facebook and Twitter
Facebook on iOS: Settings > Videos and photos > Autoplay > Use on WiFi only
Facebook on Android: Settings > Video Autoplay > WiFi only
Twitter: Settings > Video Autoplay > WiFi only
7. Track, track, track
Make sure you don't go over your data limit by downloading the free Fido My Account App and tracking your data usage on a regular basis. On Android, you can even manually set a data limit, just make sure you get the billing cycle dates right because it is very unlikely that the default dates in your settings will match your actual Fido billing cycle dates. Also, keep in mind that the data tracker in your phone should only be used as an informative tool: if you want to avoid any billing surprises, you should always rely on the Fido app as it uses the same data tracking tool that Fido uses for your billing.