Book Giveaways

I was curious if anyone in here has had any success with contests where they offered a free book giveaway. We offer this with out tours, but my tour coordinators and I were trying to figure out if this stops others from buying our clients' books if they are waiting to see if they won or not. I've had lots of responses from others who have said it didn't make a difference...they would buy the book regardless of whether they had a chance at winning, but I'm curious how it is from your perspective. Have you ever given away a copy of your book in a contest and did it thwart sales, you think?

--Dorothy T.