- Have one obvious spot to put things. For example for photos: have one directory for all of them, and make subdirectories there by either the device that took it (I remember better whether I took a photo with device A or B than whether it was in 2017 or 2016) or whatever else you prefer. Use broad, obvious categories ("photos", "audiobooks/English", "audiobooks/Dutch", "games/downloaded", "games/self-made", etc.) for the main folder names.
- Don't let data get stuck on old devices. Keep track of in which folders you've stored data and, once those are copied over, give it e.g. a few months. If you didn't need to go back to the device, wipe the device. Don't be me. I have a phone from 2012 that I still haven't finished migrating.
- Have backups. Make making a backup easy. Just take the whole disk. (Games will store your save files in places you didn't know existed.) Mark the backups as backups, so you don't have to think "eh, am I going to find any original data on this 2008 hard drive or was it just a backup copy I can wipe?". Don't be teenage me.
- When in doubt, use year-month-NAME for files and directories, then the rest of the name matters a lot less as you're likely to remember the year and season that a given thing happened in. (System timestamps are prone to being lost upon migration, or get updated when you fix a typo while reading.)
- Folders can have an 'archive' subfolder for things you may need one day, but that you aren't likely to ever touch again. I used to keep one folder per school/internship/employer, and it got a bit much. Moving old things into an archive subfolder helps and I don't have to feel like I will lose the data (it's not like I marked it for expunging).
- Have a temporary folder whose entries are prefixed with an expiry date. This contains "just in case" files, like a copy of my Signal messages database before I start messing with the database (I'll know latest in a few days that I didn't need the backup). I also put call notes here, like when someone calls me and I want to make sure I've saved the file but don't have a project folder for it, but it might turn into a project, I could put it here with some months' expiry. If nothing came of it, it can be safely deleted and I don't have to review the contents to know whether it's irrelevant.