When people refer to a site's navigation system, you usually think of some type of menu to the left or above the page. If this is all your navigation system consists of, perhaps you should consider expanding it!
The easier it is to navigate your site, and the more helpful your site is at bringing your visitors to the information they want, the more likely it is that a visitor will visit several pages in a visit and possibly even return again later. By implementing these ideas, you can increase your pageviews and repeat traffic by directing the flow of traffic through your site and to others.
WHERE YOU ARE
Assuming your site is broken down into sections, as many are, and possibly subsections, let your visitors know exactly where they are within your site. Many sites implement this idea as it will bring many visitors back to one of your main pages once they're done with the page they're on. A common way to do this is "Home > Books > Training > Manuals" if your visitor were on the manuals page. Then, they could click and return to the training or books pages.
RELATED CONTENT LIST
At the end of a page in your web site, you should list several related pages within your site. By offering content similar to the content your visitor just read, they'll be likely to continue within your site by reading some of this related content. A text link or a link and short description would be most effective.
LINKS WITHIN CONTENT
Link to other pages of your site from within your content. If you mention a topic covered on another page of your site, mentioning it with a link within the content your visitor is reading can easily compel them to visit that other page in addition to the one they're reading. This can also be useful for promoting affiliate programs related to your content.
At the end of content you could provide links to related resources outside your site. This can be setup as a link trade with another webmaster, to guide users to related affiliate programs, or simply to point out useful resources. Any way you do it, your visitors can only thank you for helping them find what they need!
There's no reason not to advertise the site your visitor is already on or another of your websites if you have more. Creating ads to run in specific sections of your site, or on specific pages, can also lead visitors back to other pages within your site. If you don't have many ads to circulate, this is another good way to keep visitors within your site.