You really have to love the NFPA standard ... 23 pages of "follow the manufacturers' directions.' Don't waste your money buying the standard.
My own thoughts? First, place it as far from the furnace as possible, and not in the path of any 'plume' of hot gas from when the burner ignites. This is, in part, because the CO detector will falsly read natural gas as CO ... and there's always some of that around when the burner lights.
Secondly, place it near where your head will be ... CO can't harm you unless you breathe it. In a living room, this is at about the 4 ft. level. In a bedroom, about 3 ft.
Don't place CO detectors in any place you can't be comfortable in, continuously, wearing a sweater. They freeze / false alarm around 30 degrees. Nor do that handle temps over 100 well.
Finally, take a marker and write ... in big letters, visible from the room, "Replace in December 2013." CO detectors have a finite, five year life. There's no fighting it ... once you unwrap it, the clock is ticking. After that period, the unit will test "OK," but will NOT detect CO.