engraving a metal is good but it is hard and not everybody can do it without proper equipment.

the best way that I have found to work for me is simply printing the private keys on a good piece of paper and then laminating it. you can also encrypt the keys with a password so nobody can read them by looking at the paper. make 2 copies and store in different places. and if you have an extremely large amount of bitcoin you can even store the paper wallet in a safety deposit box in a secure bank.