You might not think about this, but if send or receive lots of picture messages and video messages, those can accumulate and consume a lot of the storage space on your iPhone. They never go away unless you delete them manually. They’ll even travel with you from one iPhone to the next if you set up your new phone using your backup.
But how much space do they take up? In iOS 6 there was no way to tell. In iOS 7, however, we are given a bit more detail. Go to the Settings app > General > Usage. Wait for the list of apps to load (can take a few minutes). Scroll through and look for Messages. Unfortunately there is no way to delete your messages from this screen and there is no way to see which conversations are to blame. You will need to head back to your Messages app and delete suspect conversations one at a time.
How much space did this free up for you?