It's almost impossible to find a niche that doesn't have some relevant forum dedicated to it. Then all that's left is combining a high-traffic forum with a brilliant marketing plan of your own. The truth is that most people imagine this process taking much longer than it needs to. Of course, if you want to get the greatest amount of benefit from your marketing efforts you will need to dedicate a fair amount of time and attention to this endeavor. When it's your business on the line, you'll have to decide how to proceed.
One of the most important pages you can read at a forum is the one with the terms of service (TOS) on it. This is where you'll find all the things that can make your experience a long and profitable one. It's never wise to violate a forum's TOS. Doing so can get you banned from the site for life. It's not normally a matter of whether you've actually read the terms of service. They exist and you're responsible for knowing about them. This is how they see it, at any rate. If you're planning to market on the forum then you need to know and understand the terms they've established for doing so.
Forum marketing is more totally about building relationships with other members. Some marketers prefer to think of this as building a go right here network. But it's helpful to think in terms of relationships because that puts it in a better frame. People simply prefer doing business with someone they feel like they know. Everything you do and say should be done with the overall relationships in mind. If you're serious about building forum relationships you'll want to keep familiarity in mind. You have to have a visible and regular presence on the forums in order to develop relationships.
It is very much worth the time it takes to sit back and figure out who's who on the forum before you start to network on a particular forum. In general, there are a few site web people who carry a lot of sway on the boards. The membership numbers will also influence this. The longer the forums have been around and more members it has, the greater the number of people who influence the forums will be. This is something that isn't constant throughout forums. Sometimes it's the moderators who have the most influence. The 'leaders' are often the people that other members are always trying to get acknowledgment from. These are the people you want to get to know as quickly as possible on the forum. Much of this may be familiar to you if you've spent any amount of time marketing in forums. It's all about using common sense, and abiding by the rules that each forum will inevitably set up. Be a good community member and that will take you most of the way.