Pinch shoulder blades back when on the negative-side of rep.
Also, do DB. It's better at isolation of pecs anyways. I wish I could show you my progress on my pecs for the last 8 months. I should have logged it all, too. It's night and day difference. I did lots of DB incline presses @ 30* and 45* angles along with DB pullovers (back and chest movement) and stayed away from barbells for a while. Soon I will be going back to barbells to regain compound strength but so far the DB's have done me so very well.