Daniel Broadway covers some ways to deal with Green Screen Spill using some channel operations that will ultimately provide some better results that what is available in After Effects natively.
This tutorial shows how to use sophisticated channel operations to achieve a more accurate spill suppression result than is attainable with AE’s default plug-ins
Daniel does take a look at how spill suppression works in general, and shows how After Effects can handle spill suppression with its built in plugins, and then goes beyond that with some advanced methods inspired by VFX compositors in Nuke and Fusion. Daniel uses the Spill Replacer and Suppression plugins from CineGobs, which are offered as pixelbender and actual plugins (however the plugins are only available for windows systems, and the pixelbender plugins are no longer supported in After Effects CS6 [although that can be hacked a bit]). Check out the tutorial for Advanced Green Screen Spill Suppression Methods in After Effects here.