This is a topic I'm interested in, but have almost zero first-hand knowledge of.
As links and domain authority don't seem to be as important in Google Product Search as in "regular" Google Search, I think much of the "secret" might just be keyword stuffing the Google Base Attributes. I'm guessing that total word count is more important than keyword density.
If your product gets thrown into one of those "from 86 sellers" listings, you're stuck into a price competition. For that, all you can do is cut your product down to the most barebones options possible and try to upsell once the customer is on your site.
It might also be good to boost your seller ratings by offering customers spiffs for entering them, like discounts on future orders.