Pete,
I went through the same thing recently- trying to figure out a spin without either a) scaring myself to death b) killing myself or crew c) doing serious damage to the boat. After our first run I feel much more comfortable with the spin and how the boat can take it. I am very aware of the status of the mainsheet when the spin goes up. I tighten it (perhas too much) and let the traveler out a foot or so and then let go of the rope. As for the downhaul, I believe people let this out so as to descrease the compression on the mast, but I think leaving some on also helps to stabilize that axis of themast. Seems like a scary thing to do - to not hold onto the mainsheet- and if your crew is flying the spin and you feel like you need to hang on to something so you feel like you have some control- grab the traveller line. However, you will likely be surprised- as I was- how you just balance the boat with the tiller. If you feel a puff you bear off slightly and all is well. After a while it doesn't feel odd not always holding the mainsheet.