1. Don't spam one-liners.
In other words, don't reply to posts with "Great post!" and the like. That's a good way to get flagged and potentially banned.
2. Don't spam links to your website.
Including a link in a post to an irrelevant page on your site will raise all sorts of spam warnings from mom database moderators. Likewise, including a link in every reply you post (even if it's actually relevant) will result in your ban and your profile being removed.
3. You don't have to post every day.
If you want to and can provide value, please do so. But it's not a requirement. Posting once a week is more than enough to earn valuable do-follow forum backlinks.
Additional benefits of active participation in forums
It seems only fitting that I conclude this article with a section highlighting some other important benefits of being active in forums besides acquiring forum backlinks.