Print your private keys to any form of paper that you can keep for safety. Don't trust your memory even you are the champion in memory verse challenges, it isn't enough to keep your funds.
This. Even if your memory skills are top-notch, you could easily lose that skill on an accident like hitting your head hard somewhere.
You can also create a file on a notepad and save it to a flash drive and don't use that drive to any computer that you don't own. Don't use it to public places.
Huge NO. This is a huge security vulnerability. Yes, you can safely secure your private keys on your computer or a flash drive, but it's a lot lot harder to do securely than just saving it in a text file. Saving it straight off in a text file is a
BAD BAD idea. To store your keys on a computer safely, you'd have to make an air-gapped device; which is definitely not easy to do. Keep it simple. Pieces of paper on multiple secure places.