After all the (free) tutorials I've read and seen (and still getting bad keys) I stumbled across a couple of training DVDs specifically for green screen at B&H. I bought the cheapest one ($49) called "Green Screen Lighting" by a company called Reel Classroom.
Man am I glad I did, this is the most comprehensive yet easy to learn instructional video I have seen.
I had a greenscreen shoot coming up and had to learn quickly what the heck I needed to do to pull a good key. I learned what I was doing wrong in the first few chapters and I feel confident I'll never get a crappy key again.
Now I get to start watching the next half to learn the techniques of lighting talent (light angles, color temperatures etc) for the different backgrounds used to make the really believable shots!!
Randy