My website [url removed] got 2 PR by posting links on high PR websites. It fell to PR 1, when I stopped posting links. Should it be a continuous process to maintain our ranking?
Websites don't have PR, web pages. PR is per page, not per website.
Additionally, chasing PR is a huge waste of time. You rather have 100 links on low PR pages that are related to your website and rank well for the terms you want to rank well for, than high PR links from unrelated websites. PR has very little weight in Google's ranking algorithm but links from relevant websites do make a difference and are what you're really looking for.
I agree with John Conde, google really wants people to lose this focus and obsession with pr, pr should not be your focus.
If anything, constantly trying to gain links from high pr sites, especially if its from an unrelated or questionable site may look quite unatural to google.
Some of the best links are naturally occurring, ones that you didn't seek out but when other websites link back to your site and inner pages because of good written content.
PR is only one factor of many that affect ranking, and it depends what you're ranking for too, if you rank for keywords that dont bring traffic or that does'nt do what you want them to do on your site then its of little benefit to you.
From 2008 - 2010 I was building links full time for an SEO company on the west coast. The links we built had these basic requirements.
Two of the highest PR links I built and sold were on a PR4 .gov and a PR7 .gov, these were on relevant pages, contextual and one was the only outgoing link, some quotas asked for 200 PR3 contextual links on pages related to cars, when you're building links full time and by hand for 3 years, no programs and very very high quality it's extremely difficult. That said it's very much worth spending the time to build high quality links from relevant pages that have as much PR as possible.
You will see the benefits pretty quickly, I would much rather spend my time trying to build very high quality links then buying hundreds from some outsourced company where the quality is so low there is no value to them.
All that said, I would not worry about your own pages PageRank as much as I would be concerned about each pages rankings. And your conversion rates. Spend time building good links and make sure you're converting the traffic you already have