Additional tips can be found in AE Help, in Design a Breathtaking Body Shatter Effect by Lloyd Alvarez on AEtuts, and Planet Explosion from Video Copilot.
There's also a good thread with some info derived from Brian Maffitt (who originally designed this "Evolution" filter) [and update: maybe the definitive explanation in Chapter 5-6 of Brian's classic tutorial, via Shatter plug-in tour from Total Training vault].
Update: Jesse adds extra detail and corrections to his tutorial,
'I recently got some notes on my tutorial, "Custom Shatter Maps" from the creator of the "Shatter" effect, Brian Maffitt. He tells me that there are a couple little things I misinterpreted.
1) Shatter maps do not "ignore" black, they ignore transparency. In my case, the background was set to black, so what I was calling black was actually transparent.
2) "Cyan, Magenta, Yellow, Blue, Green, Red, White, and Black. Those are the only important colors to consider when making a Shatter map, and anything else will be "clamped" to those colors inside Shatter. You can turn off "interpolate palette" in Colorama to force the primaries in the shatter map, which will give you an excellent visual preview of the results."
So there are a couple notes straight from the creator. Keep them in mind when you are making your shatter maps.'