With the hope of elminating one of my many weak points, I thought I would see what sort of effective rear delt training people have done in the past, or are doing now.
Right, using the pec deck for rear delts is a great way to isolate them. Another excellent method is to do dumbell laterals, face down on an incline bench. Try doing decending sets (also known as striping sets to some) of either or both.
They may not focus on the rear delt, but I really feel it the day after. Using a rope on a cable machine, put it all the way down to the bottom and stand facing away from it. Kind of like an inverse shrug, gripping the rope behind your back and pulling up. It tends to pull your shoulders back and really focuses what I think you're shooting for.
aside from doing the reverse pec deck movement and bent over db laterals, i also do face pulls. go to the cable machine and put it up close to the top. throw on the rope attachment and place you feet close to the base of the cable machine. lean back and pull the weight towards your face with a nice squeeze at the end of the rep. eventually you'll use enough weight to where you have to place a foot against the cable machine to counterbalance yourself.