PageRank can be increased by the number of incoming links to your website from other sites on the Internet. In addition, it requires anywhere from five to seven times more to get from one level of PageRank to the next.

So for example, (and this is not a true formula) if 100 PageRank 3 sites with a link to your page give you a PageRank of 4, then it would take around 500 more PageRank 3 sites to link to you before you became PageRank 5. However if you had 1 link from a PageRank 8 site, that might be enough to give a higher PageRank all by its self. It's really all relative and trying to figure it out will simply drive you mad, so don't bother. :) The best thing you can do is obtain as many quality incoming links from other sites with similar content as you possibly can.

Another important factor is the number of pages on your site. The more pages on your site, the higher potential you have to gain in PageRank. More pages doesn't mean you will get a higher PageRank only that you won't hit a ceiling. Apparently Google has a cut off point somewhere on the highest PageRank it will give to sites with a low number of pages.