Like all streaming video services, Netflix can use a very large amount of data in a surprisingly short period of time if you're not careful. (See our YouTube article for a breakdown of how much data can be used by streaming video.)
Luckily, Netflix has some settings and features that can stretch your data a lot longer. The menus for both Android and iOS versions of Netflix are very similar, so we'll use the Android screens for examples.
First, open up your Netflix app and go to your main menu in Netflix:
And tap on App Settings to bring up the App Settings page:
Netflix has a new feature which allows you to download videos to your device to watch later. That's a great way to save data when you're away from wifi. Keep the Wi-Fi Only selection checked to ensure you don't download movies over your data plan, which would take a big bite out of your plan.
Next, tap on Cellular Data Usage to bring up this menu:
Select Off to prevent Netflix from watching movies unless you're on Wi-Fi, the best possible way to save data with Netflix.