SSH is a Swiss Army knife and Hogwart's magic wand all rolled into one simple command-line tool. As often as we use it, we sometimes forget that even our encrypted friend can be secured more than it is by default. For a full list of options to turn on and off, simply type man sshd_config to read the man page for the configuration file.

As an example, one of the first things I do is disable root login via SSH. If you open /etc/ssh/sshd_config as root, search for a line mentioning PermitRootLogin and change it to no. If you can't find a line with that option, just add it to the end. It will end up looking like:

PermitRootLogin no

Plenty of other security options are available as well. Disabling the old SSH version 1 protocol is as simple as changing (or adding):

Protocol 2, 1

Change it to:

Protocol 2

Then only the far more secure version 2 protocol will be able to connect. Every server situation has different security needs. Reading through the man page might reveal some options you never even considered before. (Note that the sshd dæmon will need to be restarted for the changes to be applied. Or, if in doubt, just reboot the computer.)