One of the easiest ways to get a relevant backlink to your own site is through making comments on related blogs. Say your niche is bodybuilding and you sell a how-to guide for building muscle.
Getting relevant backlinks is one of the most powerful things you can do for your search engine rankings – the number of links back to your site is used by the big search engines (google, yahoo, and bing). I like to mix in "relevant" and "non-relevant" backlinks from commenting because it looks more natural.
In fact, my strategy for getting backlinks through commenting on blogs is the following: Whenever I start a new website, I simply browse the sites I normally do and search as I always do. Whenever I come across a blog where I want to make a comment, I use the information from my new website. This includes my name, my URL, and my email address (those are the common fields used when filling out a comment form on a blog).
Now, many of the comments I make will be on blogs related to my niche because that's what I'm interested in at the time. But I also come across posts on mainstream media like the popular news sites like CNN, MSNBC, and Fox News. Sometimes, those sites will give you a chance at getting a backlink.
Tactically, I have two systems. I use a form-filling tool called Robo Form. In it, I can set up a profile that includes the fields mentioned above plus so much more. It's virtually unlimited in the data it can contain. When I find an article to which I want to add a comment, I simply load the profile for the website I want to get the backlink for and it fills in my information with a single click.
The second method I use is more direct. I use a tool that was specifically made for finding blogs to comment on and then post them with a click of a button. It's called Comment Kahuna and it rocks. Like Robo Form, you simply create a profile with the information above (name, URL, and email address – you can even add in a standard comment, but I suggest you refrain from doing that – make a genuine comment specific to the post and you'll do yourself and the blog author a favor). Comment Kahuna then fills in the blog comment form and you add a real comment and then press submit. You're done.
Now, the really cool thing about this software is it's free AND it actually is a search engine for finding blogs that accept comments. You can set criteria such as finding only "do follow" comments and/or a minimum PageRank level.
I will tell you that I personally don't use either of these features. I just find blogs that accept comments and I blast away. NOTE that I always read the article and only make a comment if I can add value to the discussion.
Otherwise, I leave it alone. No need in adding grief to a fellow blogger's daily chores by having to reject my comment because it's stupid or thoughtless. Plus, there are blog SPAM systems out there that can not only flag your useless comment, but it can also flag your URL or email address as a spammer. Then, you are pretty much screwed if you want to get backlinks by commenting on blogs.
So, tread lightly. There aren't many bloggers around who will mark your comments as SPAM just because you're getting a backlink from an unrelated niche IF you make a thoughtful and intelligent comment.