Watch for a section called "Top commentators" on a blog, it normally gives out dofollow links to the commentators site. Your effort depends on how many comments you need to make on the blog to get into the list(sometimes 5, sometimes 10, if you're lucky 2 comments may be enough to get your link listed). I used this method to get my link listed on 2 PR6 sites (no PR7 ones found yet).
No, not necessarily it depends on how much PR that each of those links passes. If you get a link from a high PR page that has very few links on it then that link can add a substantial amount of PR. However, most people are consistantly adding links that have a low PR and those links do help but such a small amount I think one would go crazy trying to make it on those. That is why it is best for you to go after links that have a higher PR than your site if possible. The work will be ever so much smaller. I know....I know....much easier said than done. No one can just give you a number of backlinks that will give you a certain PR. Each back link passes its own certain amount of link juice. This depends on numbers of links leaving the page and the page rank of those links comming into the page. If you want to know more about how page rank works you should look up the academic papers by Jon Klienberg and Larry Page. These were the first to tackle the subject. One thing you should note is that page rank has become a ever smaller part of the Google ranking algo. So, I would not focus all of my efforts on gathering a higher page rank unless you intend to sell links then of course page rank is what seems to matter for most.