Luke Seerveld is a friend of mine in the Bay Area. Besides being a producer he is also an LD and has a small lighting truck that we use often. A while back we were on a set and Luke was lighting a nearly head to toe green screen shot. From a post production stand point, the trick to a great key is two things.
1. Seperation
2. Green tubes in your background lights.
I want you to look at two things in these photos. In the top one Luke is standing on the talents spike mark. In the lower one he is standing back next to the backdrop in the green light. Notice he is totally “hulked out” in the shot where he is standing further back, thats becuase he is standing in the green light that is cast by the green tubes. Also, look at the waveform in the lower right corner of the monitor. No falloff! Thanks Luke!!
Luke standing on the talents mark. 
Luke standing against the backdrop in the green light.
By the way, like any GOOD LD, Luke is about 13 feet tall, hence the pose.