Hello,
after several tries making my own ink I think I found some good compromises. In terms of mixing I still wonder if a sort of shaking would help. Maybe a technique of this kind (from 0:31 on) http://www.youtube.com/results?search_query=bart+bier+sch%C3%BCtteln&aq=f
Josh has a nice blog-post about this topic where he says that keeping it in a blender for a week and mixing it every waking hour.