Article on focusing on Site Search for the "short tail" along with focusing on external search queries for the long tail of search.

If you have an internal search engine you can pick up the searcher intent for what people are looking for on your site.  Focus on making those terms convert - match the terms up with the best pages or create new ones that work better than what is in place now.

But harder yet is translating what a visitor is searching in site search with the right content on a site; it’s often not that easy to do becasue every site, every client, has it’s own problems.   According to Jennifer Laycock

Internal Searches Convert Better, Even with Generic Phrases

Another reason that I encourage small businesses to tackle the long tail for organic search is because longer, more specific phrases convert at much higher rates. That said, internal searches tend to convert at much higher rates than searches conducted on third party search engines. In fact, the article points out that users that conduct an internal site search are 2.7 times more likely to convert to buyers than site visitors that don’t use an internal search engine.

This will help even more when you have a large site; in a small site there’s not that many pages to begin with so making navigation to link them won’t be as effectual as for a large site.