James Murrays wise musings may be useful to you -
http://www.onland.info/archives/2007/12/reflected_ceiling_plans.php
I'm sure there has been lots of threads here about it also. Have you tried doing a search? These two are good places to start -
http://archicad-talk.graphisoft.com/viewtopic.php?t=13172
http://archicad-talk.graphisoft.com/viewtopic.php?p=123829