While this is seperate from making money on forums, if you have stuff you don't want anymore whether it be computer games, or text books from college, you can sell them on Amazon.com. All you have to do is create a seller account. I don't think this is a fast way to make money but you do have to have a bank account to make this work. Plus I don't know if this program works outside the USA.
Also, having ads on your site/forum/blog is another way to make money. If you'd like help with the specific company or placements, provide the link to the forum with basic stats (like visits per day/month, PR, etc).
I hope this helps you get on the start to making some basic income.
If you need more help be sure to ask around.
Andy...
I heard selling Ebooks or tutorial videos and audio files can be good at earning money if you know a lot about the topic, and have a lot of members or visitors on your website or blog which is interested in your topics. The good thing about selling Ebooks is you won't risk losing a lot of money like selling paper based books and Ebooks don't take up a lot of space since it is all digital.
Affiliate marketing can be a good way, but you really need to know how to pitch the product to convince people to buy them and how the product like "Yoga books" or service can improve their lives if you run a forum dedicated to health.
For most people, I think Adsesne, and paid per click ads are best if their site has a lot of targeted traffic.