DigiCert recommends developers take precautions when implementing their code signing process and securing the private keys associated with their signing certificates.
Maintain access controls to and accounting of code signing keys and limit their distribution. This will help enforce strict accountability for key use.
Choose a strong password for the private key. We require having at least sixteen (16) characters that are randomly generated containing uppercase letters, lowercase letters, numbers, and symbols to transport private keys. Words in a dictionary, derivatives of user-IDs, common character sequences (e.g., "123456"), proper names, geographical locations, common acronyms, slang, family member names, birthdays, etc. are not to be used.
Securely store a private key using a FIPS 140-2 Level 2 certified cryptographic device. Export of the private key is not allowed by these cryptographic devices. Most of these devices include multi-factor authentication.
Microsoft recommends using a separate Test signing certificate to sign prerelease code. The Test signing certificate should only be trusted in the test environment. Test signing certificates can be a self-signed certificate or come from an internal test CA.
For more information, Microsoft provides a best practices document on code-signing.