The Head Mistress aka Peggy Hall is the expert on marine heads and holding tank systems. Her book is available at most marine stores or online. She moderates a forum at www.catalinaowners.com select forum, then expert forum, Marine Sanitation. Here is the link http://www.sailboatowners.com/forums/menunew.tpl?fno=31 One thing to note is that Peggie does NOT recommend the use of head vent filters - they only make things WORSE!
Two projects from Capt Al to reduce head smell. Fresh water flush and a head vent filter. 
Why does the head smell?