Two-Factor Auth Using Key and Password with OpenSSH 6.2 or Higher | Now that most distros are shipping OpenSSH 6.2 or later, check out this feature to enable two-factor auth with a key and also a password!
Troubleshoot ssh Authentication Failure | ssh has a built in debug mode from the client side to provide information in troubleshooting a connection. This will provide information such as mismatch in client/server configuration options, key conflicts, permission problems, and various other useful tidbits. To connect in debug mode, simply add -vvv to the end. You can use up to three