The ideal contextual link is one that appears on a web page with high page rank. The odds that you'll secure such a link are quite small, but there are 3 ways you can get linked from sites that have high page rank (the next best thing)...
- Write a unique article and have it published on a self-publishing site
- Add comments to other people's blogs
- Help people in forums that allow you to add a contextual link back to your site from a signature
A unique article is one you wrote yourself, and that doesn't appear anywhere else on the Internet. This technique won't work with duplicate content.
Write an article relevant to your site's content, and add one contextual link back to your site. Submit it to a self-publishing site such as Ezine Articles.
Commenting on Blogs:
Find blogs that are updated frequently (2 or more times a week), and that deal with a topic relevant to your site. Look for a suitable article, and add a comment. Include a contextual link back to your site.
Make sure the Blog doesn't add a nofollow tag to your comment. You can find a list of blogs that don't add the nofollow tag here.
If you know a lot about something, find a suitable discussion forum and start being helpful. Each post you make carries your signature. Most forums allow you to add a contextual link to your signature.
Take your time with these strategies. One contextual link per article, comment or post is all you need. The more you jam in, the greater the chance you'll be seen as a spammer. If that happens, all your hard work will be undone.
If you're commenting on a blog entry, it must add something to the discussion. If it doesn't, it will be seen as spam and be deleted (i.e. offer no SEO value). The same applies to forum posts. If you have nothing to say, move on and wait until you do.
If you're going to pursue this strategy, I recommend you read this short article about links and SEO. And about the concept of creating an SEO virtuous circle.