Any tips to start a new forum??

[diamondhead]

I had been blogging since five years from now. Now, I am thinking about started a new forum in my site which is running along with my blog. But there is a problem regarding my new forum. As I ask my readers to join the forum many of them don't even visit the forum and some of them who do visit it. They didn't sign up. I was just wondering that those visitors are my regular loyal readers then why they do this? If anybody will give any suggestion regarding this then it will be very helpful.