The green screen must be approached very specifically, and this means lighting it correctly.
The Color Green
Using a green screen for chroma keying seems like a great way to transform a simple studio location into something other-worldly, but the technology has a host of limitations. Not only are you halted from creating a realistic looking situation most of the time, but getting the right balance of green is difficult. The color has to be solid and consistent all throughout the video because the computer editing software that you will be doing the chroma key in has to be able to recognize one color. What it does is replace one specific color with photo or video, which is why the green screen is a shade of green that is bright and seldom used on clothing. If there is a change in light on the green background, such as a shadowy area, then that is going to be picked up as a slightly different color and will not be properly replaced during chroma key. This is why it is so important to light the green screen properly.
The most important thing to look at is even lighting throughout. At no point do you want to have more light on one side than the other because then the image will be more visible on one side. If you have light breaks then you will have image breaks in the final product, and these are irreparable. Inspect the background carefully after lighting it to make sure that there are no spots that are unintentionally darker or lighter than the majority of the image.
Make sure to use diffused light on this, especially if you are using studio lights or a light kit of some type. Spotlights and other lights that provoke sharp fall off will no doubt create a less than even texture. Use silk screens and paper diffusers when lighting it.
When you are lighting the person or object in front of the green screen make sure they are at least seven feet away from the green screen so they do not get any green light reflection on them. Give them a slightly stronger backlight than you normally would to make sure that their color and image is separated as much as possible from that of the green screen. Make sure that their clothing and accessories to not dangle or hang off as this will create a problem when lighting them. You may want to keep all of the lights physically closer to them as a way of making sure that these lights do not hit the green screen.
The most important thing is that it looks appropriate. The background must be one color and the subject must not at all look like a part of the background. If these principles are observed then you will have better luck come editing time.