One of the best ways to tighten the security on the SSH server is to use private/public keys to securely authenticate the users and to disable the username/password logins.
The OS I chose to generate the SSH key pair is Ubuntu, but this will work under any Debian derived distribution.
To generate the key pair, run:
ssh-keygen -t rsa -C "Connect to X server from Y computer"
The -C option is to set a comment that will make the public key easier to spot between other public keys. Without setting this, the ssh-keygen command will still add a comment that will be of <user>@<hostname> format (the user you are running under, and the computer hostname used to generate the key pair).
The command above will ask you for the path to the new key pair. If you don't add your custom path, the default will generate 2 files (id_rsa and id_rsa.pub respectively) under /home/user/.ssh folder (where user is the current user's home folder).
Note: the .pub file will be the public key that you'll have to copy over to the remote SSH server. This can be done securely via ssh-copy-id command, like this:
ssh-copy-id -i /home/user/.ssh/id_rsa.pub <user>@<your-remote-host>
Read more about copying SSH keys to a remote machine.
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