backlinks
Any site filled with 100% perfect SEO/SEM will never have a Page Rank more than 3 if they dont have any other sites linking to it (called backlinks). Page Rank is a scale created by Google that ranges from 0-10. There is only 1 site with Page Rank 10 and that is Google itself. Even super popular sites like ESPN only has a Page Rank of 8. In a (perceived) logarithmic scale, 3 is considered Average Page Rank.The #1 way to increase the Page Rank of your site is to get people to link back to you by providing useful content, services, or products. You can have a site with perfect SEO/SEM and no backlinks with a Page Rank of 3, but you can also have a site filled with backlinks to it and no SEO/SEM whatsoever and this page could have a Page Rank over 5 or 6 (very popular).
There are many ways to get people to link to your site, including paid advertising on services such as Google Adwords, link exchanges, forum postings that contain signatures, and many more which are beyond the scope of this site.
Google Adwords is a particularly interesting case. Because Adwords is run by Google, Google allows their robot to follow all of the links contained within site, including the ads that they place there. If you pay to advertise on Google, not only will you receive the presentation of your site all over the world, but the Googlebot will also pickup these links and provide you with an unexpected and extremely beneficial source of backlinking. If you keep your ads up for long enough (months or years) your ads will proliferate onto higher ranking sites and these backlinks will help you immensely!