Hi jhertzberg,
we are using 3MENSIO programme - http://www.3mensio.com/ that has a module for exporting DICOM files to STL. It is sadly not free software and the export isn't that easy either.
I have spoken to a guy on thingieverse who did an DICOM - STL conversion fairly good and he said he uses Invesalius that you also mentioned. I personally have not gotten to explore that particular piece of software yet, but I plan to. OsiriX (Mac only) is suppose to do a good job too, but is not that easy to use (so I've heard).
The STL database (japanese) that Jemma is mentioning you can find here:
http://lifesciencedb.jp/bp3d/
It's a good site, with many body parts made into STL files. Not all of them though
Enjoy