Link building comes in many shapes and sizes, but one thing is always true: link campaigns should always match your unique goals. With that said, there are some popular methods that tend to work well for most campaigns. This is not an exhaustive list, so visit Moz's blog posts on link building for more detail on this topic.
Find customer and partner links
If you have partners you work with regularly, or loyal customers that love your brand, there are ways to earn links from them with relative ease. You might send out partnership badges (graphic icons that signify mutual respect), or offer to write up testimonials of their products. Both of those offer things they can display on their website along with links back to you.
Publish a blog
This content and link building strategy is so popular and valuable that it's one of the few recommended personally by the engineers at Google. Blogs have the unique ability to contribute fresh material on a consistent basis, generate conversations across the web, and earn listings and links from other blogs.
Careful, though — you should avoid low-quality guest posting just for the sake of link building. Google has advised against this and your energy is better spent elsewhere.
Create unique resources
Creating unique, high-quality resources is no easy task, but it's well worth the effort. High quality content that is promoted in the right ways can be widely shared. It can help to create pieces that have the following traits:
Creating a resource like this is a great way to attract a lot of links with one page. You could also create a highly-specific resource — without as broad of an appeal — that targeted a handful of websites. You might see a higher rate of success, but that approach isn't as scalable.
Users who see this kind of unique content often want to share it with friends, and bloggers/tech-savvy webmasters who see it will often do so through links. These high-quality, editorially earned votes are invaluable to building trust, authority, and rankings potential.