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It greatly depends on the original and how much of an effect you seek.  As one example, and probably not the best one, you can start off with the "Cutout" filter to get the basic flattening of colors so they are more easily manageable on an individual scale.  From there, processes vary, as I said in my opening sentence.  You can lasso by color or region and then put each color on a separate layer.  The better method would be to use a layer mask for each color, which takes more time and effort, but pays off in greater flexibility.  I would suggest layer masking for that reason.

Other than that, you can use the Live Trace or Live Paint feature in Illustrator to get started if you have access to that program.