mr_mischief's excellent advice aside a for a moment, I provide a bit of cold water. Fire anyone who sends user provided data directly anywhere. No taint or input validation? Fired! No placeholders for DB interaction? Fired! Also fire anyone who surfaces user data in world readable ways. Username in the cookie? Fired! Accepting GET for login forms? Fired! Cached user settings page? Fired! Let's not stop there. Someone setup a DB without a root password? Fired! Someone used the same password on all the secure entry points and it's 3 years old? Fired!
We spend a lot of time talking about difficult, non-trivial security exploits. The last two or three places I worked had holes you could drive a truck through. One place sent cookie content directly to SQL which meant a maliciously crafted cookie could delete the entire production DB.
I'm being facetious about firing everyone but I argue that the way to address this sort of problem is not more process, it's more serious consequences for not knowing how to do your job. It's a professional and cultural issue, not one caused by a lack of information. It sounds like you might be getting a handle on the cultural aspect. Good luck and keep it up!