1) Only allow login by private key pair and not by password (you don't have to disable root login then) 2) Only run the minimum amount of services. 3) Don't g

1) Only allow login by private key pair and not by password (you don't have to disable root login then) 2) Only run the minimum amount of services. 3) Don't give anyone else access to the system. (root exploits) Also be cautious if you are running some freely available software (such as phpbb). When a security hole is found, an attacker could deploy code on your machine resulting in a denial of service attack against other internet users, or in worst case, compromising your machine with a root exploit. 4) If a security exploit is found , update! If your server got compromised, make a fresh install.