Alright, Yahoo! Answers is a popular website, so it's pretty hard to advertise without getting caught. What you do is use your personal Yahoo! email address and ask a question pertaining to your forum. Then, about 3 hours later, create some stupid email like yourwebsite-.@yahoo/ymail/rocketmail.com and go to Y!A and answer the question, and post a answer, with a link to a thread as your source.
Great idea. But in this order, reply to questions asked in the niche of your forum.
For example, a guy asks a computer questions. I have a computer website, I can answer it, provide a link to my forums, and say if you need help again check it out.
I agree that it is more effective to just find questions from other users related to your niche, and answer with helpful information with your site as the source. You'll get actual visitors who are interested in the content of your site that way.