tldr; use iExplorer which provide access to iPhone filesystem
Periodically
- [ ] Periodically remove photos with iPhoto (but from my experience iPhoto does not always remove photos from iPhone)
- [ ] WhatsApp export conversation and upload to Dropbox; then delete
- [ ] SMS export using iExplorer
- [ ] Delete apps and their various data
- [ ] Remove data uploaded into some apps (e.g. movies, music apps)
Before hard resetting an iPhone
- [ ] Do some iTunes backup (but very large!)
- [ ] Sync some stuff with iCloud (contacts, notes, ..)
- [ ] Make sure some app-specific data have been backed up
After hard resetting an iPhone
- [ ] Don’t load any backup (especially iTunes) which will add loads of garbage in your phone
- [ ] Only sync with iCloud contacts/notes/..