There are many way to get free back link first of all you need to update your balog daily with relevent unique content and do Directory submission, social bookmarking, blog comment, forum posting etc
Become a member of forums discussing topics related to your site, check out which topics are currently hot and post a blog entry reflecting on those actual topics and promote your views on the forum inserting the link of your posts. On the other hand if you start writing about things people are really interested in, then chances are higher that you can get some traffic via long tail keywords anyway.
Link exchange is one of the effective ways to get real BL. But don't do that massively if you don't want your OBL high.
Submit blog on mybloglog, and Top blog directory that syndicate your rss.... it make my blog get more BL...
There is not any one technique to get backlinks, you have to do all SEO practices like bookmark, directory, link exchange, blog commenting, article submission, SMM (facebook, twitter) and many more.
As "sousen' said there are many ways to get free backlinks. You need to grow you backlinking campaign as natural as possible on Google's eyes. If you have all your backlinks from only blogs, it's not that good. You have to build variety backlinks.
article distribution and other combination would be really good but it will take some time before you actually start getting the visitors.
Adding your link to high pr squidoo lenses is a good way to build backlinks as well, and of course forums and blogs.
submit blog in blog directories and general directories. submit in blog pinging sites. get links through forum signature and blog comments