I too am very skeptical about books, DVD's, YouTube for any martial art.

You're watching the DVD, you have a mirror, you think you're doing it right ...

Trust me, you are not ... ever.

A good teacher comes over and puts you where you need to be. There, your arm goes here, your hip turns that way. Your body has to feel that, you can't watch or listen and get it, you can't think this through.

It has to be in muscle memory.

There are techniques I watch on film like before a test, watching a kata and then envisioning it. Nothing replaces training, nothing replaces a teacher.