Server security is indeed an important aspect related with an enterprise. Server comprises of confidential information for which just implementing the server is not the end of it but we also need adequate amount of security measures taken so that the server can work without any threat of hacking.
Secure passwords is really an important requirement when it is about securing your server. If an account is compromised, a hacker can make use of that for defacing the client sites or for spreading viruses. Your server can be kept secure only if your passwords are secure. It is good to avoid creating passwords with words from dictionary or with important dates.
Firewall should be set in the server with so many restrictions that you can ensure that any unauthorized access is not entertained at all in the system. There should not be any loop holes left out when you are doing configuration for firewall. The firewall should be able to monitor all the incoming as well as outgoing traffic so that necessary filtering is done and any malicious move can be isolated by the firewall. It is also suggested to disable all the daemons and services that you are not using as they allow access to the server.