I have just shot some interviews on green screen using the Sony PMW EX1.   Just wonder if anyone has any tips for keying, so it doesn't look so jaggered.

fee, you're diving in to the magic world of effects!  Get out your manuals, push that 'Help' button, and start fiddling!  You can't break anything, and you'll learn much in the process.  Your Avid has a number of ways to key.  There are secondary adjustments and garbage mattes that can help you achieve that magic, and most of it is achieved by moving sliders and seeing what they do on your screen.  Stir in some determination and patience, and you'll be the expert!

This should get you started:

http://learn.avid.com/content/tutorials/SpectraMatte/tutorial.html