Hi Simon,
I think I do similar to what Wouter describes. It realy depends on the wind strength and what your priorities are. When it's heavy I let the old sheet go as I gybe and just concentrate on getting across the boat and steering back down as the boom comes over, then pull in all the new sheet. When conditions are lighter I grab the new sheet as far forward as possible and take it across the boat with me which pulls alot of it in.
But don't worry too much, what works best for you will come with time, but if you are having trouble getting spin wrapped around the forestay, I think I would ease more sheet before the gybe. <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />