These are the link building strategy:-
1. Never buy links.
2.Content is king on the Internet, and by posting quality content, readers are more likely to mention and link back to the source on other websites, providing free links for the original content provider.
3.Create social networking accounts and provide informative posts that people will share.
4.Upload a video to YouTube, or any of the other video sharing sites. Make a tutorial, create a promotional video, or think of something quirky that will capture the viewers’ interests and include a link in the video pointing back to the main website.
5.Post on forums that are related to the websites area of interest. Interact with other users and make informative posts. Signature links can be included after a set amount of posts.
Link directories and such is good if you want to get your site indexed a bit quicker but really, in a bigger perspective SEO wise I don't think it helps your site that much. I rather focus on getting powerful links. Usually you can get links just by contacting people active in your niche, believe me, it's easier than it sounds.