Useful for international travelers(also saves battery life) disable the following:
- Background App Refresh: Settings > General > Background App Refresh
- Cellular data for automatic downloads: Settings > iTunes & App Store > and tap the Use Cellular Data switch to turn off.
- Cellular data for Safari‘s reading list: Settings > Safari > Use Cellular Data > OFF
- Cellular data for iBooks: Settings > iBooks > Use Cellular Data
- Cellular data for iCloud Drive: Settings > iCloud > iCloud Drive > Use Cellular Data
If you do not need to use data on a device you can power it off, put it in air plane mode or turn off all cellular data: Settings > Cellular on your iPhone or Settings > Cellular Data