Man of Honour
I'm trying to keep an arch in my back when benching at the moment, really focusing on that. I find having an arch seems to push your shoulders right back, and concentrates your pec activation a lot more. At least, seems to really work for me! I don't mean a massive arch, just enough that you're consciously aware that you're doing it.
Definitely, I find you hit your chest much better when you arch and bring your shoulder blades down towards your arse.
I've been enjoying some good barbell benching the last few weeks but it feels like it's caught up with my bad shoulders so back to dumbells for me.