The methods are as follows:

1.0 Comment: Read other blogs, article and posts and make a comment. Your comment should be ethical and contain a back link pointing at your site. This very common method for building a back link and many bloggers do this to maintain back links.

2.0 Article Marketing: Article marketing is a very useful way to create back link. You can write free article at article base or ezine articles. This article will help you generate free one way back link. Write a article about a topic that is covered in one of your posts. Then create a link to your site as a reference. Allure your reader to go through the link pointing at your site. So when your article will be published it will contain a back link pointing to your site. It is easy, isn’t it?

3.0 Forum Signature: If you read and post  at forums regularly, it is expected that a lot of people see your posts. That is a lot of people see your signature. Keep your site link in your signature. It will give you some traffic and it will be back link to your site as well.

4.0 Do not spam: I receive a lot of spam comment with some URL. This kind of spamming to create one way back link is not ethical and can damage your web popularity. If you are marked as a spammer, no one will accept you as a friend without spammer.

So keep building your back link. All the best.