Definitely second the Darrell Gholar dvd. It's a stripped-down approach to takedowns, both on the mat and up against the cage. For a more comprehensive dvd series on fundamentals, takedowns, takedown defense, tie-ups, and throws (might be some trips in there too, it's a lot of information on 6 dvds) check out Ultimate Takedowns with Mike VanArsdale and Kevin Jackson. It is such a great reference, but it can be a little overwhelming too. Just pick a handful of stuff you like and work on it. There's just too much info otherwise. That being said, it's more takedown and takedown defense heavy (2 dvds each), so there may be better dvds out there on fundamental techniques and principles.