There's a tendency in this link-happy world we're living in to include loads of links in your online text. And don't get me wrong -- I love links. But you need to use them wisely. Too many links create a sort of link fatigue, and many readers just ignore them altogether. Some tips:
1. Link to improve understanding or deepen knowledge.
Linking to examples, explanations or evidence (data) readers might need to have a full understanding of the concept is useful. But if you find you have too many terms that require this additional supporting information, you may want to think about offering some of this information inline or creating a glossary or FAQ. This also is a good way for you link relevant archival content.
2. Link to give access to tools and resources.
A link to download related content (clearly marked as such) has a higher likelihood of getting a click-through than a link that appears to lead only to random content. This is another way to leverage older content that remains timely. It's also useful to link folks to customer service reps or an engaging discussions happening in your community forums should they want more information.
3. Link to promote partners and advertisers.
If you don't mind giving visitors an excuse to leave your site, a few strategic links to partners and advertisers can add value to your content. If you're talking about a product you love, including a link (or affiliate link, so marked), makes a lot of sense and doesn't feel pushy. You also may want to link to some relevant sites/blogs we love.
So the next time you think you want to link, consider your readers. If the link would be of value to them (really, not in your own mind), then put it in. If not, skip it. Same goes for "relevant content" links you might use in a side bar.