I agree that a steamer is best. You must be careful, however, that you don't hold the plate against the wall for too long as the heat will cause the plaster skim will lift as well as the paper. I have also found that when using a steamer as you go round the room removing the paper it gets easier. I assume this is due to room getting full of steam/moisture and soaking in to the remaining paper. Therefore an even easier way might be to fill the room with boiling kettles until the paper almost falls off by itself with a bit of luck. Doubt if I'd ever try this myself though.