The Spanner

Ronald has an excellent article on secure programming:-
Secure programming flowchart

The reason I like this article so much is because it introduces a way of thinking. I generally think like that, I have a list of steps in my head and I try to code my scripts with various steps of security. The further you go the more secure your application will be, it is important to note that each step must be carefully be monitored in order to avoid security mistakes. We all make mistakes, so in order to reduce the risk of exploitation make sure that the damage is limited on each of your steps.