Building backlinks for any kind of site is always a challenge, getting the backlinks to stick is always the hardest part to creating backlinks but also findin sites of a similar niche to which you can place those backlinks so that they become relevant is also a task, especially if you meet a site owner who is not keen on having your link placed on their site.
I found that the easiest way to create good quality, relevant backlinks was to become active on other forums and blogs that were similar to my niche and having the link to my forum in either my profile or signature. Guest blog posting is also a great way to increase relevant backlinks for your forum as long as the blog post is informative and does not look like spam it will work.
What methods do you find work best for building backlinks for a forum? Are there any kind of methods you would avoid?