There are many way to increase backlink including article submission, directory submission, blog commenting, forum posting, link exchange etc
Yep, you can comment on dofollow blogs, participate in forums, write guest posts, these are good and powerfull ways to to earn some relevant good backlinks. and its natural to Google, so will have good effect on your SERPS
What i like the most is buy link from a very high pr site. Which just 10 pr 5 site will increase my site to around pr 3 too.