Tasei Corporation from Japan has developed a system of dismantling skyscrapers from the top down, first removing structural elements by hand (having obviously propped the roof) and then jacking the roof down. It is called the Tasei Ecological Reproduction System.
The process apparently leads to improved recycling of building materials, and reduced carbon dioxide emissions by as much as 85%. It is also reported to significantly reduce dust emissions in the neighbouring area.
Tasei Corporation: http://www.taisei.co.jp/english/csr/hinsitu/jirei_hinsitu.html