I've finally just started rails this last week. I've gotten pretty cozy on boxes and then got decent with down boxes.
Rails scared the shit out of me though for some reason. Best advice I got was my friend told me to just start pretending the edge of the box was actually a rail and focus on keeping balanced over the edge and weight downhill.
Did this a few times. Went to rail... Actually thought it was easier than the box. Went to down rail and pretended I was just hitting the side of down box... Brilliant success!
It's funny, I was so scared for such a silly reason. It seems when I mess up on a rail, I just sort of slide off and if I fall, it's onto the snow. With boxes, I go down and I go down hard on the box.
In regards to the spinning off thing. I'm probably doing it wrong, but I've been trying to get cozy going on unnatural so when I start spinning I can go FS off. Just need to get my arms under control...