Forum marketing can be a bit tricky. You have to be subtle or you can get banned. Look below at the "long post" for a list of the forums I have joined. It is a good idea to read and post in each of them every day which I have been really lax on. Now that I am not trying to publish my articles every day I can start posting there again.
Forums can be a lot of fun because they allow for open communication.
Make a fun avatar
Say something cute and memorable in your signature and you will get a good and friendly reputation. Steer clear of political and religious comments and threads and if you choose to post something there be sure to be extremely tactful!
If someone starts to goad you to stop and don't respond at all unless you can be super professional. Read the whole thread looking for tips and opinions that can help your business. Then post if you have something meaningful to say.
Never post an ad on a forum!
Make sure that your signature has your link and talk about what you have learned or how your life is changed by "your opportunity" but don't use phrases like "Come Check this out!" or "Come to my site"
If you find someone who you think (from reading their posts) would be a good member of your team send them a PM and don't invite them to join up. Just say hi. Mention that you enjoy reading their posts and would like to be friends. Give them your username or your email address.
After you get to know them a bit they will likely ask you what your opportunity is or for sure they will check out your site. You likely won't even need to invite them. Go to theirs and if they list a blog link add it to your blog link.
Be always on the lookout for people who you can connect with. Maybe they have a service that would help you market your site or maybe they are just fun people to get to know. Maybe they would benefit from your encouragement.