The most important thing to remember when creating an optimization strategy is that Google Shopping Ads don't run off keywords in the same way that normal PPC marketing does.
Optimizing your product feed requires a focus on product titles, categories, and the images themselves to improve your return. SInce you're not keyword bidding, you must use keywords to signal the relevance of your ad to specific search parameters. The same principles governing SEO also rule paid search.
Here are 10 optimization tactics for success in your Google Shopping campaigns.