I have recently improved this by introducing BIP 38 and a working proof-of-concept implementation of it.
https://en.bitcoin.it/wiki/BIP_0038Instead of keeping a whole private key (51+ characters), you can instead keep just the password/passphrase of your choice, which serves as the second factor.
This is a whole lot easier to keep around. You can engrave it on a gold-plated bar, or include it on a small card inside the presentation box of a gold coin. Best yet, it uses a very slow variant of "scrypt", so even an 8 character password will provide a decent amount of security. It only has to be kept secret from the maker of the coin (me), and can be publicly displayed to everyone else... A Bitcoin-related phrase could be used as a passphrase that displays well, such as "Money of the 21st century"