How do I make my dog overcome her fear?

Okay so we got a dog from a shelter in August 2009. She is awesome and we all love her :) One thing is that she has a horrible fear of fly swatters. Whenever we take one out (which we do a lot in the summer) she goes and hides under a table. We think that someone hit her with a fly swatter before she came to us. We tried putting the swatter down with a treat on it but she wouldnt take it. What can I do to help her overcome her fear?