A Year With Symfony: Writing healthy, reusable Symfony2 code

I've written A Year With Symfony for you, a developer who will work with Symfony2 for more than a month (and probably more than a year). You may have started reading your way through the official documentation ("The Book"), the cookbook, some blogs, or an online tutorial. You know now how to create a Symfony2 application, with routing, controllers, entities or documents, Twig templates and maybe some unit tests. But after these basic steps, some concerns will raise about...

• The reusability of your code - How should you structure your code to make it reusable in a future project? Or even in the same project, but with a different view or in a console command?
• The quality of the internal API you have knowingly or unknowingly created - What can you do to ensure that your team members will understand your code, and will use it in the way it was meant to be used? How can you make your code flexible enough to be used in situations resembling the one you wrote it for?
• The level of security of your application - Symfony2 and Doctrine seem to automatically make you invulnerable for well-known attacks on your web application, like XSS, CSRF and SQL injection attacks. But can you completely rely on the framework? And what steps should you take to fix some of the remaining issues?
• The inner workings of Symfony2 - When you take one step further from creating just controllers and views, you will soon need to know more about the HttpKernel which is the heart of a Symfony2 application. How does it know what controller should be used, and which template? And how can you override any decision that's made while handling a request?