Warning: PageRank is a moving target - what was written here today may not be true tomorrow or next week. use it as a baseline and conduct your own research afterwords. Heck, I may have even gotten a thing or two wrong here (I'm only human afterall). Speaking of links, don't forget that PageRank is all about links. Google has a preference for natural links (like the one I gave in the above paragraph) and will give them more weight if used properly. The more natural links pointing to your Web page, the higher your page rank will probably be. The more links pointing from your page to other pages, the lower your page rank will most likely be. Ideally, you want MORE links pointing to your page than you want pointing out. Bear in mind this works for EACH page, not your whole site or domain. Google doesn't care about your whole site's PR (which doesn't exist) - it weighs pages, not sites, after all. One thing I like to do, and this has far more to to do with usability than search engine optimization (the search engines will benefit from the human-first approach, which confuses many people and convinces them that the search engines prefer this method when they really don't care either way) is to include natural, relevant links to other pages of my Web site in my page content. I'm not talking about the menus, or a site map here. I mean organic links, like the one to the PHP/SSI tutorial I wrote last year. Obviously, the link would lead to a page on my site that was related to (and relevant to) the page I'm linking from (for example, if my page is about teaching people how to review movies, and I have a review of "Pirates of Caribbean 3" elsewhere on my site, I'd probably link to the page as an example of how to write an effective movie review), rather than something totally unrelated (like vacation trips to the Caribbean). What I'd be doing is giving some weight (or credibility) to the Web page I was linking to. While this will bleed some PR (PageRank) from the current page, if you link your own pages into a "web" of pages (responsibly, don't over-do it), it'll balance out in the end, and you may (doesn't mean it always will) even enjoy a net profit on your individual pages' PR before the external links to your pages start coming in. And please, for the love of all that is good, don't use nofollow on links - you're doing the owner of the page you're linking to more harm than good, so if you do, you might as well just use plain text anyway and avoid linking altogether.
Page Rank is as important to search as a Thermometer is to the Weather. Until you figure that out... well you are just spinning your wheels.