Duncan, make sure when you shoot it, that the subject is far enough in front of the green screen that none of the green light bounces onto their shoulders and hair.
Also, if you use the "color key" filter you can use the dropper to select the exact color. If the screen was not lit well and one part is a darker shade than another, use the dropper to select the color. Make your adjustments for that, then add that same filte to your shot again and select the darker area which did not key the first time.