There are a few things that you should know about marketing on forums so you do not get in any trouble at all and get the best responses out of its members. First thing is Private Messages, Members get a private message and they feel special, but to get the best response out of them is to ask a question in your first Private message. Leave your signature making them wonder who you are. When they wonder who you are they will like to check out your website(that will be included in your signature) just to see who you are.
There are many forums and many different product you can use to market on all theses forums. Get into it and I bet you can make over a full time income just from marketing to all kinds of forums.
I first came across this one great product that i saw posted in another forum, It is a Private Messenger bot, To tell you the truth this is the only Private messenger bot i have ever seen. One of the first. But i purchased it, Tried it out and got drastic results with it. Im surprised how easy this is to get back tons of results. Program is called PMBlaster.com. Works with all Vbulletin and Phpbb forums
There are just so many different marketing ways out there today that there are so many competitions. But at least doing private messages there are very low competition and you get the best results.