I use an old iPhone supplied by my employer so it does not have tons of storage space.
When I receive messages that my storage space is running low I go to my iTunes app and try to rent a really big movie (I generally choose the Lord of the Rings trilogy because that is enormous). My phone then tries to clear out space to download it, before giving up and letting me know that there is insufficient storage. I generally do this two or three times, which frees up to 5G of storage without deleting a single thing on my phone. And without me having to actually download anything.