Use green fabric for your backdrop, rather than poster board. Fabric, especially cotton, is a good choice if you are doing the lighting yourself because it is less likely to deflect light or cause a glare. You'll also find that fabric can be folded up and more easily transported than large sheets or poster board.
Measure the distance between your subject and the screen. You should allow 10 to15 feet of space in between the two so the subject does not cast a shadow.
Illuminate the green screen backdrop on its own with a "key light," or the main light on your set. Before you add in props or actors, you want to make sure you have achieved even lighting across the entire screen. You will want even lighting for the digital backdrop that will replace it.
Use another source of light to shoot your subject in front of the screen so that it has a clearly defined outline. You can turn off the light that illuminates the background in order to see your subject's outline more clearly. This will avoid any blending or bleeding of the background onto the subject.
Check that any objects or actors who will appear in front of the screen are not the same color as the background. Taking this extra step will save time during the post-production process, since all things that are the color of the screen will essentially disappear.