If you are a living, breathing human with an iPhone, you've probably run out of space at some point.
Redditor eavesdroppingyou discovered that if you try to rent a movie from iTunes, your iPhone will automagically erase some files you don't need.
First, check your storage. You need to have mere *megabytes* of space left for this to work.
Next, open iTunes and try to rent a super-long movie in HD (to make sure the file's as big as possible). Then, when the pop-up appears, press Settings.
Some suggestions: Pearl Harbor, The Lord of the Rings, Pirates of the Carribbean: At World's End, and Watchmen. More super-long movies here!
Now CHECK OUT YOUR STORAGE. We were able to free up over a gigabyte!
Finally, repeat the process a few times.
For us, it worked the first, second, and third time. On the fourth try, no more storage could be removed. :(