private keys in written form in my personal diary is the safest way to keep your private keys. Saving in Computer or laptop is risky because if it gets damaged you will lose all so best if to keep your private keys in written form.
No brain is sharper than any pen, this is one thing I learned the hard way you can be the best person out there in terms of having an excellent memory but you'll never know. It's best that you have a back up ( I'd go something in writing ) for you not to forget it.