I?d recommend you to research top competitors? sites in the first place and learn where they got their backlinks from. As soon as you reveal competitor backlinks, you will be able to make use of their backlink strategy. Pro-level software allows distinguishing between backlinks coming from blogs, forums, social media sites etc. Having this information obtained, you may pick the appropriate websites, blogs, directories etc. and use them for building your own backlink profile.
Apart from that, you may continue with researching your niche further and find out which blogs and forums are niche authorities, and then plug in there with guest posts, comments and posts. Moreover, use article directories, reviews and press releases, web 2.0 sites, Q&A sites for submitting your content to, since many of them provide their services for free.