We only show sitelinks for results when we think they'll be useful to the user. If the structure of your site doesn't allow our algorithms to find good sitelinks, or we don't think that the sitelinks for your site are relevant for the user's query, we won't show them.
At the moment, sitelinks are automated. We're always working to improve our sitelinks algorithms, and we may incorporate webmaster input in the future. There are best practices you can follow, however, to improve the quality of your sitelinks. For example, for your site's internal links, make sure you use anchor text and alt text that's informative, compact, and avoids repetition.
Google currently supports rich snippets for people, events, reviews, products, recipes, and breadcrumb navigation, and you can use the new schema.org markup for these types, just as with our regular markup formats. Because we’re always working to expand our functionality and improve the relevance and presentation of our search results, schema.org contains many new types that Google may use in future applications.