I use the ol' plate on the face thing. I do 3 sets with a plate on my face (padded by a towel of course ) and then three sets lying face down balancing the plate on the back of my head. Balance is a bitch, but I prefer it to machines or isometrics. The harness is good to work the sides more specifically whereas it seems pretty tough to do the same with weight plates.
I have tried all the above and they work ok. Try "bridging" using a swiss ball a try. I find nothing works better than this. Also doing snatch grip Hi-pulls works the upper traps very well also.
The last time I found them was about a year ago on the muscle and fitness web site. I can't find my print out now, but maybe you can still find it on the site. www.muscle-fitness.com