I think that with most things a ‘back link’ has to be relevant. Just plastering links to your site all over the web could be seen to annoy the world. Just use your common sense.
If posting in a forum, and have a relevant link to a page on your website then take the opportunity and link to it. Even if the link is ‘nofollow’ people are likely to click it. If they like it and they are more likely to be a happy customer – and may spread the word.
This forum has sent a few people to my website before. In some other cases @chriscoyier has picked out a few good posts from the more inspiration posters in this forum and made articles from them.