Well, Page Rank is the importance of your site to Google... while SERP is for traffic or possible client try to focus more on SERP and not on page rank.
Join forums, forums are a great way to achieve links to your website. In most forums you are allowed to have a signature and in your signature you can put a link to your website. But another important note to look on is making sure the forum is somewhat related to your website. You will still get credit if it's not, but if it's related to your website than you will be accomplishing two tasks at once.
Try getting Links form the relevant sites to your site. Try building a good content useful site and there are chances that people will link ("Vote") for you.