I realize this is an old question, but thought I'd note for the record that as of May 2012, Google has announced that they are doing away with Product Search and essentially implementing PPC ads in its place. So if you continue to use what used to be Google Shopping, you will need to place bids in order to get your products to show up at the top of the results, much like AdWords works.
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RE: Does anyone have any tips for optimizing your Google Product Feeds?