As an absolute layman, here is a trick I was told and it seems to work well.
and it is borne out here
but wait there is more inlcluding steak knives.
i forgot one more thing.
twist your back foot so your heel is pointing more towards the nose of the board.
there, i'm spent. i've spewed everything i know about sailing upwind. i think.... 
now if only i can get rid of this feeling of needing to have a sail. 
A waist harness that can twist around a bit (or a hook that can move... or Gestalt's double hook spreader) are all good ways of getting your body pointing more towards the nose of the board, which helps when going upwind dramatically.
I wear a rashie over my wetsuit so that my harness can slip around...
Perhaps boardies would help if you wear a seat harness? [}:)]
yeah, the sliding spreader is the way. allows you to really rake the sail and twist your body forward.
my double hook spreader is the bomb for going upwind. i ended up ditching it though as the broad reaches were a little scary at time when really powered up.
It works! 
I used a variety of tips from Guy Cribb..
www.guycribb.com/userfiles/documents/cranking%20upwind.pdf
+ suggestions here & got miles upwind.. I even found myself a bit bewildered at having to go off the wind so much to get back..It did help that it was blowing enough to plane ..mostly..gusty 15 - 20kts with 105lt B & J + 5m sail..
Have to post another topic on harness lines though as mine seem too long to pull the sail on properly..
Main things were sinking the windward rail ,foot position & positioning the mast towards the eye of the wind rather than raking it back like i used to..(80's tips re closing the gap)
i modify every harness i buy so harness hook will slide laterally.
a narrow bar will increase this lateral movement even more.
great for getting max weight to nose of board in gusts + for upwind sailing.
unable to sail properly with fixed bar.
this for me is the biggest kept secret in w/surfing.
have no pics. costs around $20 to modify. make sure u use top 2 buckles only/disregard bottom buckles. use narrow bar, increases distance bar will slide.
