Google's Explanation of Sitelinks:
The links shown below are some sites in our search results, called sitelinks, are meant to help users navigate your site. Our systems analyze the link structure of your site to find shortcuts that will save users time and allow them to quickly find the information they're looking for.
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 completely automated. We're always working to improve our sitelinks algorithms, and we may incorporate webmaster input in the future.
Yes, for many of us (including myself) Google Sitelinks is a huge milestone, an ultimate goal, and the proof of superior SEO skills. Only the best sites seem to get them. And getting them appears to have a very positive effect on site's visibility, popularity and reputation.
Google is not sharing any information on how Sitelinks are populated, so we have to look for subtle clues and trends to see why the sites with Sitelinks get them.
Here are some factors that sites with Sitelinks seem to have in common:
#1 Organic Position in Google
The site holds # 1 position in Google SERPS for the search term generating Sitelinks
The older the better, but the sites with Sitelinks all seem to be at least 2 years old. Older domains always get more trust from the search engine. Unfortunately, there's nothing you can do about your domain age except for wait. Here we are, back on the topic of trust.
Company name word(s) matching the words in URL
Not sure how important this really is, but notice how Toyota, Amazon and CNN all have Sitelinks.
Well structured site navigation that appears to be easy to index
Relatively high traffic from organic search results and high click-through rate on for the search terms
The number of links pointing to the site's index page - it also seems to be important that inbound links are from the reputable sites.
What can be done to increase possibility of getting Google Sitelinks?
Promote your site. Site with low traffic and few inbound are not likely to get Sitelinks.
Be selective about outgoing (outbound) links on your site. Make sure they link to quality content.
If your site is very young, there's not much you can do, but renewing your domain name for at least 3 years may help in the future. At least you are planning on sticking around.
It appears that Sitelinks are much more important to larger businesses or organizations with significant brand penetration compared to smaller businesses. If someone searched for "Great Point Designs", they obviously already know about us. Personally, I am much more interested in having the users find us by searching for the services we provide. However, if Google gives you a nice Sitelink-enhanced listing for at least one of your search terms, you know you did a great job as a webmaster. There's something about the look of that listing in the SEARPS that seems to put a lot of trust into consumers' hearts.