I just created a post in my blog on how to get back links form blog commenting. I will highlight some important points in the post.
Some people are not get links from their comment because their comments probably did not get approved. To increase the chance of your comment being approved, you need to
1- Comment with your real name.When you fail to use your real name and stuff keywords as your name, then you just made yourself to look like a spammer.
2- Upload your picture as an avatar. If you use default image as your avatar, the blog owner may consider you to be a spammer.
3- Reading the post will help to get your comment approved. For your comment to be approved, it has to be meaningful; you can only contribute meaningful comment when you have read the post.
4- Avoid random words like nice post, great post. That is what is common with spammers, for you not to look like one, comment something meaningful.
5- Avoid dropping links in your comment, bloggers are not comfortable with it.
6- Be the first to comment.

http://www.uniqueblogging.com/using-blog-commenting-for-backlinks-the-right-way/