Link building is a full time work/job. Link building requires careful planning. The link building techniques I use are:
1. Content Creation: Content has always been the best way to link back to your Web site, either on your site or posting as a visitor on someone else site. Other Web site owners will link to you without even asking; if your content is very good. Make your content pre-sell, and people will link back to you without asking.
2. Write for Search Engines.
3. Use online PR. This is different from paid online service
There are also many more methods, but these few will boost your site.