Google Sitelinks are generated by Google from site contents in order to help users navigate the site in a better way. Google generally shows sitelinks for results which they think, will be useful to the users. Google generates these Site Links periodically and also takes various site factors into consideration such as the site architecture, site navigations, the number of links pointing to the site with the main keywords as the anchor text, the number of searches and click through rates for the main keywords, the number of pages indexed for a particular keyword, the age of the website etc.
Site links can be achieved by following certain SEO techniques and tactics in the most proper way. There is no particular process which can give you a guaranteed site links in Google. But there are certain things that we can follow in order to get sitelinks.
Some of the steps that I find useful are
1) Easy site navigation structure.
2) Increase in traffic to the site
3) Proper SEO of the site.
4) Maintaining stable top rankings for the major keywords
5) Getting quality backlinks.
6) Updated XML sitemap generation (once in a month)
7) Using Google Webmasters tool.
8) Strengthening the internal links.
Moreover the age of the Domain is also considered. Google doesn’t give site links to relatively new websites. Follow the above mentioned things and wait till you get site links. Eventhough you cannot make Google Sitelinks happen yourself, you can control the links to be displayed in Site links via Google Webmaster Tool. I have tried the above mentioned things and was able to get sitelinks to 95% of my clients.