At its core, SEO is about building high quality links from a large number of websites. Link analysis reveals how pages are related. With metrics like trust, spam, and authority, search engines can determine how important a particular URL is and assign it the appropriate ranking.
In the SEO industry, professional SEO practitioners believe link-based factors make up a considerable portion of a search engine’s algorithm. The volume of links is important, but link building should first and foremost focus on acquiring links from trustworthy websites. Experts link to other experts.
Short of buying links, how do you acquire natural links on other websites? SEOMoz has a great blog entry about actual emails you can use for link building.
The blog entry addresses everything from guest posting to acquiring links on .edu sites. The actual examples of emails used provides you a template for creating your own emails to acquire links.