SEOmoz | How to rank well in google products search & a big list of places to get reviews
How to Rank Well in Google Products Search & a Big List of Places to Get Reviews
Provides a list of ranking factors for Merchant Center. By far the most important is having reviews. Provides a list of 3rd party review sites, but the one with the most weight for ranking are Google Checkout reviews.
SEOmoz post that breaks down how to get the best rankings in Google Shopping (Google Base, Google Product Search).