Just thought I'd share something that we learned in a google groups thread, which has helped us to greatly reduce the spin we see in our flight videos.
In that thread, James with the University of Minnesota suggested fixing our payload to the neck of the balloon. That's because now our payload and balloon rotate together, which means for the payload to spin, the entire balloon must spin. Thinking back to the formula for moment of inertia, the balloon has a massive radius (I=(2/3)MR^2 for a hollow sphere), giving it a very high moment of inertia compared to the payload. (Admittedly, it also has a lot more surface area.)
We have done two practice flights with this technique. Comparing the spin rates to previous flights using a single line and several swivels, there is a noticeable advantage to fixing the payload to the neck of the balloon. We are trying to improve on our initial design and and develop a better rigging system, which I'll try to photograph this weekend.
Is anyone else trying to reduce spin, and how is it working out for you?