Other than what was already suggested above, limit your app installations to your most essential or most used apps ONLY. Uninstall unnecessary third-party apps which you barely or not use at all.
16 GB storage is abysmal by today's standards especially since apps/games are getting bigger in size thus requiring more storage space (plus incremental app/game updates & fixes). Best thing for you is to buy/upgrade to a higher capacity phone model and keep your old one as a backup phone.