iOS 7 seems to be eating my free storage space. The only sure-fire solution I've found is to backup, restore as new, then restore the backup to the phone. I've recovered over 3 GB of space this way. That's an awful lot of wasted space, and I think the glitchiness of iOS 7 is partly to blame here (though this has always been somewhat of an issue with the iPhone - it just seems worse in iOS 7).