Just make two shots from the same point (tripod). One with empty scene and one with somebody lying on a chair. Then combine these two shots in some graph editor and erase the chair on the upper layer (it will show the floor or other background from below layer). Thats it! :)
If you check out this guy, he has done several of these type of shots along with lots of other technical stuff and usually gives behind the scenes shots on flickr along with tutorials on his website. Very good stuff, as he is very knowledgeable.
http://365project.org/louish/profile