I agree with Jordan here completely.
I started Kickbox with Alain Bissanthe's Kickbox Circuit from the Body bar company. It is excellent for beginners, intermediates, and he also does muscle conditioning for kickboxing, slows down the kicks to show you the exact mechaanics of them, before you speed them up for the cardio blasts.
Also, people like Janis Saffell, so you could try her tapes from Collage video.
Clare