"How do I get the best Chroma Key possible?"

The simple answer is to do the best lighting job you can. Here is one way to light a Chroma Key scene:

1. The background screen should be lit as evenly as possible and independantly of the talent.
2. Your talent should cast no shadows on the screen.
3. Your talent should have 3 light sources on them :
   
   • The main light (place just to the side of the camera - brightest light).
   • The fill light - so talent has no dark areas/shadows (place on other side of camera - dimmer).
   • The back light - to light the edges of the talent (place behind talent).
   The main light is what you generally think of as "lighting".
   The fill light is what keeps your talent from having drop-out areas on them.
   The back light is what cleanly separates the talent from the background.