For Debian Squeeze configuration file is located in /etc/ssh/sshd_config and at the end of all the changes that will need to restart the server.
1. First step to one more secured ssh server is:
Change the standard port for ssh server
The first safety rule is to change the default port because the majority of automated tools to perform brute force or dictionary attacks right at this port.
In the sshd_config file change the port directive to anoter port. I recomend to use a port above 1024.
Port 22222 or some other port
2. The second change is:
Disable root access
To disable direct root connection set the following option:
3. And the las change will be:
Allow only some users or groups to connect to server
For only a few users can use the directive:
For groups use the following: