On the web somewhere, I can’t recall where, I found this trick to recover space on your iOS device. Check how much space you’ve left in Settings-> General, in my case I was left with 800 MB. Now open the iTunes Store, go in movies and select a movie to rent greater than the space you have free, let’s say 4GB. Select to rent it, after a few seconds you’ll get the message that you don’t have enough space and told to manage the free space and you’ll be sent again in the settings. Check the free space you have now, it should be increased by several hundred MBs; in my case went from 800 MB to 1.2 GB. Repeat this again four or five times, until you see that the free space doesn’t increase anymore. In my case I ended up with 2.1 GB free.
I do not know exactly what iOS does to free the space, I guess it will clean some cache or the like, but I would have never been able to clean 1.3 GB of free space manually myself.