Almost every three months once google will update their page rank value towards all the websites available on the Internet. They have just declared that their google bot search tracking reach 1 trillion urls. It is reasonable time to take 3 months to give page rank value for each urls.
Illegal Page Rank Distribution
1. The more incoming link from higher PR (DoFollow type of links) to your url, the more credits you will earn to your page rank calculation on next update. This does not mean by buying links. I don’t know how Google finds paid links, but it is illegal to purchase any links with dofollow. If you buy any paid post with dofollow links, or paid to be in dofollow blogroll then you might get the penalty for your existing page rank.
2. Try to get more back links legally. There are tons of dofollow comments blogs which giving backlinks to your comment. You can start to search on google image seach for “U Comment I Follow” and get it list out. Also, there are many giving dofollow 10 Top Commentators widget. Participate in commenting on these blogs and get your name listed on the widget. This is legal page rank gains which will give positive effect towards your PR.
3. If you are earning something from the links you posted on your blog, like links in paid posts to the advertisers, affiliates links which earning you directly money from your visitors purchase, links in blogroll. There is nothing wrong to give a link to the advertisers who paid to be in our blog. But, it is illegal to give dofollow link to them. So, if you are giving links from your blog for paid links, please NoFollow the links to avoid getting penalty.
Legal Page Rank Distribution
Paid links should NOT DOFOLLOW (MUST NOFOLLOW).As long as you follow this rules, you are legally distributing your links with page rank juices.