I\'ll throw in my comments, as well, on the glaze medium (even though they\'re not entirely new or original, but whatever).
This stuff is pretty neat. I\'ve found that it slows the drying time just a touch, too, so in addition to making your paints a bit more transparent, it will act like a bit of a retarder.
I did an experiment with this stuff, trying to build on the \"underpainting\" technique that Eric Louchard first mentioned in an article on this site. I did a write up with the full details as well; just check the articles section.