Forums are a long term strategy, meaning you have to start building your presence there long before you actually want the backlink. So just start joining relevant forums and post a few there every week not worrying about backlinks. Do this in your spare time and over the months you will have established yourself very well. Make sure these forums are do follow. There is a good chance you will also be able to generate solid traffic from those who click on your signature link.
As for what else you can do, have you considered guest blogging? And, I'm not talking about spinning out articles for publication on other sites, but really writing a good article and submitting it to a high quality, high page rank blog that is similar to yours. If what you have written is in depth and worth reading, then you will deserve the backlink. And, that one really good backlink will be worth ten times what a forum backlink is worth.