Share 'Let's talk about giveaways.'

I have avoided doing a "giveaway" on my blog just because I did not want to screw it up and tick off my readers. But I realize it is a great way to build loyalty and get rid of my pile of "Already Read, but Don't Really Need to Keep" books. My question is this, how do you fairly determine a winner? Any other Do's & Don't would be aprreciated, too.