Removing apps, photos and messages you don't need / use. You could put photos on iCloud or Google Drive instead of on your device.
System data can be quite a storage hogger at times, as can be old app data. The only way really to clear this is a fresh iOS install.
With apps...have a look at which ones are taking up the most space. Sometimes, deleting the app and installing it again can clear stuff up (it did for me with Facebook, for example).
I used to use an iOS cache cleaner software on via Windows a few years ago but it has disappeared now.