Here's some advice I offered on another thread:
First apply the Reduce DV Artifacts in Browse Presets> Image - Utilites. It'll blur out any color subsampling blockiness. (I'm assuming you shot this on something that records to h.264)
To get a really good basic key, there's only a couple things you need to mess with on Keylight: Screen Matte> clip black and clip white.
First use the eye dropper to select your green.
Switch the view mode to Screen Matte. Adjust both so that your talent is completely white inside and the green background is completely black. You can also use View>Status for an exaggerated view of this (Black is 100% transparent, white is 100% opaque, and gray areas are translucent. Green areas are also receiving despill correction.)
And that's pretty much it to get a really good basic key.
You might want to check out Primatte 5 for After Effects. It's a more intuitive keyer that has a surprisingly good one-click "auto compute" mode. You can also draw on the screen to tell it what parts to keep and what parts to key out. Also, DVMatte is an inexpensive fast keyer designed for crummy low-res footage.
There are a creative cow tutorial using color key and choke and then keylight that also produces really good results. http://library.creativecow.net/articles/rabinowitz_aharon/junk_mattes.php
Errr, it must be bad, I was using Primatte, I don't like the auto-compute because with the lower quality video my camera takes (working on this) it never gets it right or sometimes even close. One problem I was having is while cleaning the background, after so many selections while viewing the matte it stops selecting what I am clicking on, leaving the extra artifacts you see floating around, especially in the opening clip. I haven't tried DVMatte, I'll give that a look.