I've only really optimised for Google Products in Google Base - so this has a destinct google products feel to it.
I've done a little bit of optimisation with Google Base which I have blogged about here:
Your Product Feed Appearing In Google Base But Not Google Products? How To Fix It
Generally, it depends on what part of Google Base you want to appear in (Generally - Products is the most common). Often inbuilt feed generators in e-commerce software do miss out on a number of things (for example, if you sell things in 50grams, 100 grams and 150grams, you don't want to be ranked for those three terms, more "Product Name 50grams" - however one popular ecommerce solution by default misses out the Product Name).
It's quite an easy thing to do though, as the feeds are well structured XML files, so open it up in Microsoft Excel you can add keywords to the title and description tags for products. Generally it's the same as basic SEO - product name as the main keywords & manufacturer, product description'll contain the weight/colour etc.
This seems to work well, though it doesn't always help. one of our clients has a wonderfully optimised feed in a popular niche, but is £5 more expensive than most
