http://forums.adobe.com/thread/435659?tstart=0
"Pre-compose the keyed layer (you can duplicate before Pre-Composing if you don't want to compromise the original).
You are only pre-composing it because some effects that will take the key as source wouldn't see the key but the original unkeyed footage as result if it wasn't in a Pre-comp. This pre-comp can be turned off. You don't need to see it. The effects need to see it Apply the Set Matte effect in the solid. Make sure it points to the Pre-Comp in the "Take Matte from" menu, and uses Alpha Channel in the "Use for Matte" menu. This will mask the solid with the key. Then apply either the Simple Choker or Matte Choker effect (Effects > Matte) to the solid, and tweak the choke so that it closes the holes. This will give you a solid that is roughly masked like the keyed layer" -- Adolfo Rozenfeld
you can freely employ any adjustment and trickery to ramp up contrast between foreground and background no matter how strange it may make your footage look. Remember, that it's easy to recombine color and Alpha using Matte modes or effects like Set Channels/ Shift Channels/ Set Matte -- Mylenium
