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There are many factors involved in getting a good key, and a search of this site and the web in general will yield a lot of good tips.
A few things that I have found -
1) LIGHTING - light the background and subject separately. Get a nice even light across green screen, and make sure you use a fill light from above and slightly behind subject, to light their hair and shoulders. This separates them better from the background. Keep subject away from the green background to minimize "spill" of green reflection hitting them. If green edges on subject is a problem for you after trying above steps, add a magenta gel to the fill light to suppress the green on head and shoulders.
2) CAMERA/FORMAT - most camcorders today highly compress the video, and worse yet, throw away a lot of color info, recording 4:2:0 color. Much better to record 4:2:2 color, but not many cameras do. A device like the Atomos Ninja 2 can take the live HDMI feed from camera (prior to compression step) and record to 4:2:2 using the ProRes codec for better keying. You can also use a computer with a Matrox MXO2, BlackMagic Intensity, AJA, etc, capture card and record live camera feed that way, again using a codec with 4:2:2 color and a much higher data rate (less compression).
If using a DV camera, that format just doesn't provide the best keying and you can only take it so far. I've had better results with HD cameras, and better yet with 4:2:2 record options when possible.
Safe Harbor Computers
The only things I can add to Mr. Pulera's excellent advice, is to get some distance between your green screen and your subject. This helps cut down on green spill onto your subject. And as Mr. Pulera says, always use a back light on your subject. Finally, make your green screen as wrinkle-free and as evenly illuminated as possible.
and I would add...you need to use all the features of the KEY FX to help pull the key.
eg, edge feathering, softening, spill,...
Sometimes you need to use the Key Effect twice on the same footage and often a garbage matte helps as well.