Jim Boykin has a nice post about Optimizing Your Internal Link Structure. In addition to providing background on how page rank flows from page to page, Jim outlines the basics on how you should prioritize which pages should get the most attention.
Keeping in mind that your site as a whole only has a finite amount of page rank, you need to decide how to focus it. In essence take you most important keywords, make sure you have pages for those, and give them the most internal link love. If like most sites, your home page has the most page rank, link to your most important pages from there, or even consider giving them a site wide link from your site.
By clearly communicating what you believe your most important pages are, the search engine will do a better job for you in ranking those pages. Note that this is likely something you would want to do with your users too (i.e. steer them to your most important pages). Whenever you find yourself doing something that is good for both users and search engines, like optimizing your internal link structure, you are truly in the sweet spot of SEO.
Links, Internal Links, Optimization, How To