On a USB drive or other data storage medium in a safe place (e.g. safe deposit box, safe)

1. On a paper wallet.
2. On a bearer item such as a physical bitcoin.
3. Use a offline Bitcoin Hardware wallet.

Bitcoin cold storage is achieved when Bitcoin private keys are created and stored in a secure offline environment. Cold storage is important for anyone with bitcoin holdings. Online computers are vulnerable to hackers and should not be used to store a significant amount of bitcoins.