Disable wifi when you are not using it
Prior to leaving port download monitoring app like: DataMan Next, smartsapp, etc.
Prior to leaving port consider lightweight browsers like: Opera Mini or Text Browser<\li>
Keep apps closed, as some will actively try and update/download when they are open.
Settings > iTunes & app store > Disable non-essential apps (make sure not in low power mode to access option)
Settings > general > Disable background refresh
Settings > iCloud > disable iCloud for non-essential updates (specifically pictures and drive) OR settings > user > iCloud > disable non-essential apps
Settings > podcasts > disable podcast downloads
Settings > news > disable podcast downloads
Settings > Mail > accounts > Disable fetch new data
Settings > Mail > disable Load Remote Images
Settings > Messages > Low Quality Image Mode (for iMessage)
Chrome > Settings > Data Saver
Facebook - Disable videos autoplay: More > Settings > Account Settings > Videos and Photos > Video * Settings > Auto-play > Never Auto-plays Videos
Twitter - Disable videos autoplay: Gear > Settings > Data > Video Autoplay > Never play videos automatically
Instagram - Disable videos autoplay: Gear > Mobile Data Use > Use Less Data