Just like everybody says, backlinks. However, don't focus on PR, focus on quality traffic. Unlike many people saying that PR is irrelevant, it still is a small part of the algorithm. It helps increase the spider crawling of site pages for example. Google doesn't seem to care about PR much anymore, toolbar PR for instance didn't seem to be even updated since April 2 when before it used to be updated every 3 months. Focus on traffic and relevant quality links instead, pr will follow. Its shouln't be your goal, it should be a byproduct of your efforts. The goal is to maximize the relevant and quality traffic.
Hello friend, To increase PR you make the quality content then join to forums,submit your site into search engine directories,create and submit your article.you can include a signature in your article.and do other offsite activities to get more backlinks which help to increase PR.
only solid backlinks can improve your PR but if you write the genuine content than you will get natural backlinks that are far better
Many say content + PR. In fact content has nothing to do with PR. PR increases with the number of quality inbound links. It is purely based on back links.
Google hasn't been updating the "public PR" for a while now. Maybe they'll stop updating it completely, thus making it irrelevant. You should focus more on getting SERPs.