Your question is too general for specific answers. Are we referring to physical security, network security.
How secure do you want your network to be? Keeping in mind that more security means less accessiblity and ease of use..
The weakest link in any computer/network is the user.. do you for instance, only allow a user to logon from a machine with a specific MAC address during a specific time period.. and only has access to a restricted range of programs installed locally.. ensuring that no removable storage is available and USB/Firewire is disabled in the hardware to prevent someone from simply walking in to a screensaver passworded system and inserting a USB thumbdrive and disabling the screensaver?
How physically secure is your site.. do you allow wireless at all on your site and do you monitor for rogue access points being installed on the network??
Do you keep all machines patched with the latest security updates to prevent 0 day exploits? There is a multitude of software that accomplishes this task.
So many questions, so little answers..