The most worthwhile forums to post on are the private forums where you have to register or even pay to be a member. Most forums will have a reasonable code of conduct and there will be an etiquette for members who post on the site.
The benefit of posting on most forums is that you are usually allowed to add a signature to the bottom of your post and this signature file may be allowed to have your web site URL included.
The more discussions you take part in, the more copies of your signature file will be on the website and the more traffic you will drive. You do want to avoid getting into an argument on a forum. Most forums have certain people that just want to cause controversy and have no intentions of offering constructive advice to anyone. I think personally they just like to see their name 'Up in lights'.
You should only post if you know the answer, there is absolutely no point in posting something like 'I don't know the answer either'. Just keep quiet if you don't know the answer and if you think you do know, just double check on the Internet that you do in fact know what you are talking about before posting a reply.
Only reply to the current day's questions; there is no point posting answers to preceding day's questions as the chances are the person has found the answer to his problem or is no longer interested in the solution. Either way, answering older questions is a waste of time.
Don't spend time gossiping or in idle chit chat, get on the forum, pick a few questions you know the answer to or can easily find, answer them and go back to productive work. You won't make money if you are just passing the time.
Do not just answer the question with a short sentence, offer a comprehensive answer that is knowledgeable and completely answers the problem. It is far better to answer a few questions with a good comprehensive answer than lots of questions with a short simple answer.
Answering questions also helps you in search engine positioning as your name comes up in posts related to the questions you have answered. You could also ask the forum owner if you could become one of his resident experts where people ask you specific questions related to the forum topics. You could even write regular posts, much like blog posts for the website.