Writing articles for other websites in your niche and including a link back to your site in your author bio or within the content.
Finding broken links on other websites and reaching out to the site owner to suggest a replacement link, which could be a link to your own site.
Creating high-quality content that is better than what's already out there on a specific topic, and then reaching out to websites that have linked to similar content and asking them to link to your improved version.
Creating visually appealing and informative infographics and promoting them to other websites in your industry that might be interested in sharing them with their audience, along with a link back to your site.
Finding instances where external sites have linked to broken pages on your site and reach out to ask them to update the link to point to a live page on your site.