First of all you must be lacking in high PR backlinks because If you have back links from high PR and quality number of sites then you can get more and more high PR for your site.
Provide great content, provide a good user experience, be different and solve peoples problems. Promote via SEO and Social Media and eventually after much hard work you will acquire some links. Link popularity = pagerank. Try not to obsess over pagerank and put that energy and effort into creating something that has value and creates buzz. Good luck.
Get links one way or another. There are ways to get a few high pagerank links directly from big sites... for example make sure you set-up a google profile and include a link to your website. Another great way is to comment in forums or under blogs and use your website in your signature or as the url they ask for.. just make sure it's a related post. Whatever way you do it, it will probably take at least 3 months to get to a decent pagerank(that's if you do about an hour a day or posting comments etc.).
POST COMMENTS ON BLOGS, GET LINKS FROM DIRECTORIES, Link Exchange or Link Swapping , etc worked for me. Hope works for all..
Links form sites that have high PR or form .edu sites. But be sure that the links are DoFollow! In link bellow you have PR table, so if you get 1 link form PR6 site your site will have PR2,..... http://www.softdivision.com/info.php?info=114&stranica=menu
PR is not very important now? Good and useful content for your visitor. Visior oriented site improving...
Inbound links will build your PR and you will get a boost. Ignore the rubbish about do-follow and no-follow ( its mythical ) as I know loads of top ranking sites with no-follow. use automated software to build links quickly
Links from pages with high PR & internal links from some domains with high PR (social bookmarking sites, blogs, etc.) will help with future PR updates too. Directories, blogs & articles are all nice methods which people already mentioned but if you have some good content that people would link to naturally that's probably one of the best ways to get a good PR. If you can find some sort of way to go viral you will get hundreds or thousands of links, but it's definitely not an easy thing to do. Some things I could think of off the top of my head would be sponsoring some web templates themes (pay to have your link added to footer of template by designers links) or link directory templates. An interesting Youtube video that people would naturally link to or some other type of free tool or service. If you can get lots of people linking to you naturally your site will get a big boost in PR and search results as well. (if you used keywords in your links) For high PR links you could sign up for services like webshots or livejournal, etc. and post comments on pictures, articles, etc. Many of their pages have PR5 dofollow links which are pretty easy and most are auto approved. Don't spam them though you will get banned. Vary up your anchor text so Google gives more SERP juice to your unique keyword links. Any other suggestions that haven't been mentioned?