I researched and believe this is the best answer:
To do freelensing best, use:
- a prime lens in the medium focal length range 35-85mm
- large aperture to capture plenty of light
- hard infinity-focus stop
- for Nikon lenses, a non-G lens as the G lens stops all the way down when not attached to the body
- for Nikon bodies, Canon lenses are supposed to work very well
From what I've read on Flickr and dpreview, this is the best lens for freelensing: 50mm f/1.4 (for Nikon the non G version 50mm f/1.4 AF-D is best).