The best method for a clean key is light on the screen most important. Separation of the talent from the screen is the best way to keep bleed off from occurring. However I've worked on tight "portable" setups where this is difficult. In these cases, I suggest two 300's (or stronger depending on your needs) with magenta gels (this is key, pun intended) set behind the talent facing the talent. This will help wash off any green spill on their hair / shoulders / halo.