There are quite a number of demonstration videos (either tape or DVD) available. Many of these are videos of Guruji that are interesting, but not something that many people can follow along with. There are also a number that relate to teaching yoga; I haven't paid much attention to those, but I gather that they are various classes taught by Guruji, generally for the benefit of Iyengar teachers who are learning more about teaching.

One Iyengar teacher who has made several introductory level videos is Patricia Walden.  I believe that her videos are available on this (yoga.com) web site and also through Amazon.com.  I've seen clips from these, and her practice is truly exceptional. She's also an extremely well-regarded instructor. So, you might check out those vids.

For myself, I watch most yoga videos just once, to see what I can learn.  I can't practice to a video, since I tend to get lost in what I'm feeling.  The next pose comes as soon as I am ready for it, and no sooner.  I day-dream a lot too...as you can imagine, I'm a dreadful student.