Perhaps you are syncing to iCloud and not moving the files to iCloud. If you just sync, they stay on the phone. But, if you move them to iCloud you'll have extra space. You can also move those files onto a PC or Mac so that you have another copy and free up space on your phone.
Use cloud based apps like Box, Dropbox and OneDrive to store photos and videos. Then delete them from your camera roll. Delete messsges from the messages app. They take a lot of space if you don't delete them. Delete apps you don't use. Delete music you don't need. Delete app cache where you can. Dome, like Facebook, have to be deleted to get rid of the cache. Then install them back. Delete emails. Good luck.