Switch off data roaming if you're leaving the country
Unless you're set up with a decent international plan, it's a good move to disable data roaming altogether so you don't accidentally rack up some ungodly data fees while traveling abroad. Doing so won't affect how your phone works in your home country, but will prevent you from using the Internet sans Wi-Fi internationally. Go to Settings > Cellular > Roaming and tap the Data Roaming toggle button off.
Go nuclear and shut off cellular data altogether
If you've already hit your data limit and can’t stand the thought of paying a dime more on overages, go cold turkey and cut your data off completely. Go to Settings > Cellular and disable Cellular Data. It means you won’t be able to Snapchat or browse Instagram if you leave your Wi-Fi zone, but you also won’t be on the hook for extra money and can focus on more fulfilling activities, like face-to-face conversation.