The most effective way of leaving comments is to surround your link with content. When you leave comment, do not comment just to write "great post" or "just continue 'or something so similar. Write at least two or three sentences, the search engine will value more and the owner of the blog.
Another thing: I noticed that some blogs get me more visits than others. I research to see why this is so, to see what this blogs have that other do not. I found that these blogs have the 'top commentators' option, which you can now see on my blog to. The point is to leave as much comments, so that your link is in side bar, so that people can see it, regardless of which page on your blog can be found. If you feel that you will never catch person who is in the first place, there is no need to worry, because this option reset after a certain number of posts. I set my blog to reset when someone reaches 30 comments, then all start from the beginning.
Get as much blogs with this option, and regularly leave comments, after a while you'll see that the number of visits to your blog increasing.