Now that you have some idea of what you're up against, you can start thinking of how you want to approach the problems. The easiest thing to do is look over the preceding items and identify what your current configuration is weakest against. You'll also want to identify what your organization considers the most important points and data to protect. Once that's done, it's a good idea to perform some sort of an inventory in an attempt to discover sensitive points that may not have made the list for some reason or another. Sometimes, this can be done simply by asking questions such as "What would the impact be if someone saw that file?".
At all times, it's important to remember that there is no way a system can be truly secured without making it completely inaccessible to anyone. If even one person can get into the system, it's also possible for someone else. Computer security is not a one-time event; it is an ongoing process of re-evaluation.
It's also important to remember that computers are just machines. No matter how advanced the hardware and software is, the computer does not think. If an instruction makes it all the way to the CPU, it won't stop to ponder if the user or program that submitted it should be allowed to do so. It won't consider the moral implications of carrying out the instruction. It will simply do as it's told. Security is a human endeavor.
This book advocates both for taking specific steps to secure specific systems and for a defense in depth approach. The defense in depth style recognizes that not all attacks can be known or planned for in advance, so it attempts to mitigate them by using a layered strategy. If the firewall is penetrated, an internal network access control list may halt a break-in. If that doesn't work, intrusion prevention software may stop the attack. If that also fails, a simple password challenge may keep the intruder out.