When you go to your Amazon library, or if it's your first time watching something on Amazon, at the bottom of your screen you will see a row of icons. The one on the far right is the little gear icon for Settings. Click on that, and you will see a list. In the middle section the list is "Streaming Settings," "Download Settings," and "Autoplay." Set your streaming and download settings to "Good." Set your autoplay to "Off."
It makes a HUGE difference! The "good" setting will use up to 0.6 GB for every hour watched, while the "Better" setting will use up to 1.8 GB, and the "Best" setting will use up to 5.8 GB for every hour watched!
That means if you have 15 GB anytime data per month, and your Amazon is set to "best," you can watch fewer than 3 hours of streaming video before you run out of data. With the "good" setting, you could watch up to 25 hours of video streaming before you run out, assuming the only thing you did on your device was watching streaming video. YMMV depending upon what else you use your device for.
These numbers will be similar for Netflix, Hulu, YouTube, etc. Monitoring your data use at least daily will help you use it wisely, so it lasts all month.
Turning autoplay off prevents Amazon from automatically starting the next episode of a TV season you have purchased.
:-)