Even though security flaws made at the design stage may be more costly and difficult to fix after the software is written, it is at the implementation stage that the "last chance for safety" occurs. Clearly, there are a great many things that you must keep in mind when implementing your design. Coding a software design is a process that requires a great deal of caution and care, and there is no substitute for experience. And even the experts often get it wrong!
We all can learn from how things are done in other industries. In the aviation industry, practitioners make extensive use of checklists, in addition to training pilots on an ongoing basis on how to properly operate their aircraft. They also intensely analyze the mistakes of others; whenever an accident takes place, the Federal Aviation Administration (here in the U.S.?no doubt other countries have similar practices) distributes a causal analysis memo to all pilots, for mandatory reading. We all are well advised to learn from that model. Study all the information that you can find; pedantically use checklists at each step of the development process; test and retest every aspect of your software.
Never assume that you have stopped learning how to implement secure code. Oh, and if you must make mistakes in your software, at least be original!