I hate to sound cliche, but there are really only two ways to get useful backlinks. First, write useful, engaging content that people want to read and tell others about. A little controversy can help engage people too. This has been said a million times, but I really feel there is no real shortcut or secret that works long term.

Second, comment on others blog posts, but only if you add meaningful on-topic comments related to the post. You may find that the blog writer will visit your site to see what you are all about, and if they like what they see and it's in their niche, they will link to you.

I have to say, the majority of comments on my blog are spam. Nothing but spam, and I delete them immediately. Blog writers are wasting their time with generic spammy comments such as "I like your blog and added it to my rss reader" that is crap, pure and simple, and gets deleted by me and most other professional blog writers.