Link building is very important part of Search engine optimization . Link building help to getting good organic traffic to website.
classified posting etc.
Add quality content in Article submission, forum posting, guest posting etc.Keep in mind about Google latest updates and do changes according to that updates.Avoid black hat seo technique,keyword stuffing,duplicate content etc.Follow Google guide lines.
If I talk about link building strategies now there isn't much left but what you can go forward with is the forums (you can get backlink from the signature), Yahoo answers, classifieds......and yes one of the most important thing that you need to keep in mind while working out with the link building process, link link relevancy must be entertained.