Checklist

A backup checklist you can actually follow

No single step here fixes everything on its own. Together, they cover most of what causes people to actually lose files.

Before anything goes wrong

Make a list of the files you would actually be upset to lose — photos, documents, creative work — rather than trying to protect everything equally.
Keep at least one copy of important files somewhere other than the device you use every day.
Write down, somewhere you will actually find it later, which service or drive holds your main backup.
Test that you can actually open a backed-up file, not just that a copy exists.

Treat syncing and backing up as two different jobs — a synced folder updates everywhere instantly, including when you delete something by mistake.
Give files names that describe their content and date, instead of relying on memory or "final_v2" style naming.
Empty a device's trash or recycle bin on a schedule you control, not automatically the moment you forget it exists.
Check every few months that automatic backups or sync are still actually running, not silently stalled.

Before retiring an old device, confirm every folder you care about has been copied off it and opened successfully on the new one.
Export your data from a service in an open format before closing an account there, if the service offers that option.
Keep a short written note of which accounts and services hold copies of your data, so a device swap does not mean starting from zero.
Give yourself a week of overlap between an old setup and a new one before wiping anything.

Save important documents in widely supported, openable formats rather than formats tied to one specific program.
Revisit old backups every year or two to confirm the storage medium itself is still readable.
Keep more than one copy of anything irreplaceable, on more than one type of storage, in more than one place.
Note the date next to any backup you make, so a future version of you can tell how current it is at a glance.