So, I'm about two months away from releasing my next book

, and I'm sorting through my distribution/marketing options. I'm obliged to offer signed paperbacks because people have already claimed a spot on the preorder list even though I haven't actually started a list yet.

So while thinking about preorder lists and talking with a friend about ordering incentives, I had an idea...
With Red Rain, I opened up preorders as soon as I knew how much the book would cost. This was mostly out of necessity - I wasn't ordering many extra hard copies, so if you wanted a signed one I had to know ahead of time. There was no incentive for preordering; the price was the same. This go-round, however, I will probably be keeping more books in stock for other purposes. So preordering isn't a necessity but rather a courtesy, almost as a way to raise some of the printing funds beforehand. So I was wondering... perhaps it would be a good idea to offer some kind of incentive for preordering, to encourage early sales?
So the question is two-fold... Is an incentive a good idea, and if so, what?
Offering a discount and/or free shipping is the obvious option, although I won't know how feasible that is until I can calculate the cost of the book. A free ebook would make sense but doesn't seem very special to me. I will be selling an audiobook album later, so that's a possibility. Of course, collaborating with someone else to offer a discount on their product (and them offering a discount on mine) would be awesome, if anyone was game.
Ideas?