You can find as many forums as you want that are relevant to your niche. All that's needed now is to find the forums that get the lion's share of the traffic in your niche so you can put your plan into motion. This is a plan you can put into place fairly quickly, no matter what you've been led to believe in the past. With all good things, however, time and care are required in order to get a maximum ROI. Only you can decide what works best for you.
Most people, maybe everyone, behave in forums according to their general personality. Most people are going to be either aggressive or shy. So just be yourself and let people see your real personality unless it's highly offensive. If you are not an aggressive person, then you can just look around before you feel confident enough to say something. This is something called lurking that most forum members do at the very beginning. It can feel a little daunting to make your first post, and it's a lot like being in a new city or town and going to a new school. Just post in other threads and add something to the conversation.
Take a quick peek at the signatures if you want to ascertain if marketing is okay. Determine if the signatures possess links in them. This is what is done if forum marketing is okay. Once you visit enough forums, you'll be able to tell pretty quickly what the prevailing rules are for members doing marketing in that forum. In addition, you can pretty much tell what you can do by studying the signatures of other members. For instance, you can often see multiple links with graphics, etc. This will tell you whether or not marketing is really allowed.
Figure out who has a lot of influence on the forums before you really dig into your own networking efforts. You'll typically find there's really a handful of people who have this potential. There is web some influence to this based on the sheer number of forum members too. Forums with long histories and large memberships will have more influence bringers than smaller forums. This is something that isn't constant throughout forums. Sometimes it's the moderators who have the most influence. Usually a senior moderator who's been there forever and many people basically try to win favor and acceptance. Once you've identified them, it's time to start building relationship with these members.
So think of forum marketing as another form of social marketing. And it is easy to see that forum marketing is just another type of social marketing. They are both based upon relationship marketing and you should become very familiar with it.