Haven't read any of the former replies, so I will just tell you what I do before I squat. I do stiff leg deadlifts, but I only use a moderate weight and focus on stretching the hamies out by standing on a block of wood. This has helped me immensely with butt wink, but it hasn't eliminated it completely. I now focus on going to the depth that my butt wink kicks in (which is about half way between parallel and ATG) and than powering up from there, it is a bit weird because I am very use to going ATG with my squats in a high bar Oly squat, but now I am in between a Oly squat and a power squat. My legs are slightly wider, and I don't go quite as low, but I'm working on it. I still go way below parallel, so I am still going deep. Worth a try mate.
EDIT: OK, I read the thread and hip flexor issues may be the cause, I am going to try some of the execises also....Thanks Dave