i just realized that whenever im sculpting using layer intensively (going back and forth between layers), the active memory become insanely huge although i only have 6 million polygon. Whats up with that? 
Layers can take up a lot of memory because potentially ZBrush needs to store the data for 6 million polygons for each layer. Add to that undo history and memory use can soon increase dramatically.
any viable solution? or restarting zbrush every 30 mintues or so is the only one
Preferences>Mem>Compact Now will help. And also, if you can reduce the number of Undos in the Preferences>Undo History that will help also.