A large number of free backlinks means higher rankings in Google, but also to a large number of directories. Manually done, you can lots of free bakclinks, but an incredible amount of time (10 minutes presentation, 1500 filing of Mathematics).
Then, to obtain a reasonable number of free links, you have two options:
You can pay by writing articles, and directory services to collect the fees for each shipment. So much for the link for free!
The problem is that these costs will be repeated when the new campaign begins, as everything you need to build links to your blog and website, so if you are then free link is not the best way to go.
I've tried a variety of special services, such as Isnare (article) and the Maximizer folder (directory) links and get through it is great because you do not need to do, but they can be expensive and limited. In addition, you are not 100% control in order to get free backlinks.