David Swenson is an incredibly well-known yoga instructor and practitioner. He began practicing when he was 13 and was introduced to Ashtanga at 19 by David Williams and Nancy Gilgoff (also very well known). Plus he studied with K. Pattabhi Jois, the master of Ashtanga.
His DVD, Ashtanga Yoga: First Series, available through Amazon for $24.95, is wonderful whether you are first learning or have been practicing for years.
There's a 30 minute intro explaining the foundations of the practice. Immediately following is the complete fully guided First Series, in the traditional flowing rhythm essential to Ashtanga Yoga. Ending with a guided deep relaxation, the series is about 1 1/2 hours long.
David's delivery of the basic principles is clear and down-to-earth. It's easy to follow along, and I love that he offers modifications so all levels of yogis will be able to follow along.
I definitely recommend this video if you want to learn First Series, or if you've already fallen in love with it. It's challenging both physically and mentally, yet very rewarding.