Dr Andy,
I use a rock climbing harness and a 'beaner clip. What I do is I have a loop of static rope that I wrap around the tree and back through the loop creating a cinch. I then clip my beaner into the tag loop and I'm hooked in before I get in the stand. When exiting I do the reverse and can even leave the loop around the tree for a return trip. Temporary knots can be made in the loop to adjust length. I'm toying with installing a line to put a prussic knot on during ascent so that I am always tied in.