A common mistake with green screen, I think, is that people try to make the green really bright. You want good color saturation, but making the screen as bright as you can will result in more green spill on your subjects and make for a harder keying job in general, I think.
Light and expose for your talent first and then light the green. You should aim for about 50% – 60% on the IRE scale. If you’re not using a scope, you can still estimate something that’s well lit but not super-bright. The screen shouldn’t be brighter than the talent.
Also, subject back lights should have some kind of color gel, like gold or amber, that helps seperate their edges more than plain white light would.