Get it right. This exercise isn't just plug-and-play, so to speak. Proper alignment is key. With your chest flush against the pad and hands on the handles, your upper arms should be parallel to the floor at shoulder level. This means that you will either need to adjust the seat to the ideal height, or stand slightly, to achieve proper stimulation of the rear delt. From this position pull back in a wide arc and hold the peak contraction for a count. Don't focus on your grip -- think about slamming your shoulder blades together in order to keep the focus on the rear delts.
Build it in. If your rear delts are lagging, then you need to bump them up in your shoulder routine. Relegating them to last, as is often the case, usually results in last-place development. After a few sets of heavy, overhead presses, get right into the reverse pec deck. Save your stronger delt heads -- the lateral and anterior heads -- for last.
Rep right. It's a myth that smaller muscle groups require lighter weight and higher reps. One or two "heavier" sets will likely provide a new stimulus, which is sure to elicit a response from your rear delts. Try three sets of 8-10 to failure, then on the third set, do an extended drop set, moving the pin down one notch at a time and repping to failure at each stop.