I think most giveaways are retarded, since most sponsors never do them in a way that nearly any webmaster can win, they are always tilted to where only a top select few have a real chance at winning.
Now to create real excitement think of a lottery style contest of some sort that even a small time webmaster who sent you 10 sales could win realistically and then you have something that everyone will want in and will benefit your program as well.
