I touched on the topic of tying necktie knots in an earlier entry. Thanks to a recent StyleForum thread, I'm linking to a couple of useful visual tutorials to tying bow ties in particular.
The first is a Washington Post slide show which is especially thorough and specific in its explanation.
The second is a short video clip illustrating how to tie bow ties with adjustable neck collars:
VideoJug: How To Tie A Bow Tie