I really like the advice Global Moxie has given their users for how best to promote their content management system, Big Medium. In their blog post, Psst... Pass It On! Spreading the Word about Big Medium, they talk about various way users can "spread the word" about Big Medium. They show a good understanding for how online communities work, including what you shouldn't do when marketing a product via a blog or forum.
Don’t be a zealot
If you’re posting in forums or on blogs, be considerate of the community around you. Offer comments that are appropriate to the context and that will genuinely help people understand if Big Medium is right for them. No software is perfect for every project, so feel free to share Big Medium’s pros and cons. Be yourself, be honest.
One of the frustrations I have with forums sponsored by the various CMS owners and open source projects is when no other CMS but their own is seen as a solution. Potential users of CMS have a difficult time knowing what to believe is the right application for them when no one admits that their product isn't right for them.
Take the advice to heart, don't be a zealot when selling your product. Nothing is perfect in this world so why would you sell your product as such? Based on your honesty when maketing your products, perhaps you'll find you have more users based on the trust relationship you established with users.