My products are listed on Google Shopping but don't show up in results.
There are two sides to the Google Shopping listings (and google natural search listings for that matter). Getting ON the listings at all, then rising to the top of search results.
Once your products are successfully inserted into Google's system, typical SEO best-practices come into play. Some practical pointers...
1. Front-load your keywords in your item names. Google looks at and displays the first 70 characters of your item names so be sure to load your item names keeping your keywords intact and up front.
2. Front-load your keywords in your descriptions. Google weighs the early text in your descriptions more heavily than text later on the page. Keep focused on what keywords and keyphrases you are targeting and use them early and often (not oversaturated, but emphasized).
3. Load UPC, Manufacturer-part-number, Brand, Price and other descriptive data on your products. Google craves this information so making it easy for them to grab it means your products are more likely to float up high in search engine results.
4. Get site reviews. Lots of them. Use rating services like Bizrate to garner reviews. Google reemphasized their want for this information as a means of improving their customer experience. If Google wants it, give it to them.
5. Watch out for duplicate content. Set up a canonical tag on your Yahoo store. To quote my programmers, this is a no-brainer. The canonical tag tells Google and other engines that when a 3rd party site links to your pages and they have a tracking suffix on the link, it is NOT a separate page on the web but actually a link to YOUR page. By clarifying the inbound link is not a separate page, you avoid being penalized by Google for what they think is duplicated content.
6. Watch your inbound links. Don't use link farms. If Google thinks you have links from unsavory sources, they will potentially delist your products. If they don't delist, they may still demote your products way down on search results pages. Contact sites that Google considers unsavory and have them remove link references to your site. If they won't respond or won't remove the links, Google has a doc you can use to disavow connection with the source site.
7. IMPORTANT: Use an SEO / SEM firm. Get guidance from search optimization pros. These folks keep abreast of all the algorithm trends and can guide you on ways to keep your content updated and your site code fresh.