You can find as many forums as you want that are relevant to your niche. If these forums also manage to garner a fair share of traffic, it's a simple matter to capitalize on that traffic in a really big way. The entire plan doesn't have to take nearly as long as you might imagine. 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. Only you can decide what works best for you.
Your first step is to establish a positive reputation for yourself in the forum and as an expert in your niche. If you want to enjoy the best possible results this is a necessity for you. Of course, no one is going to recognize you if you aren't able to be seen first. What you want to avoid is spending a lot of time in any forum. This, after all, is only one part of a much larger marketing strategy.
But coming in and taking over with a large number of posts in a short amount of time will be counter-productive. Figure out when the most members are present and time your posts during those peak times. Make sure the posts you do provide are quality posts rather than simply confirming what others are saying or going repeating points already made.
Understand that forum marketing is, first and foremost, about building relationships. It can also be viewed as building a network. In this instance, it's wiser to focus on building relationships; not networks. Right or wrong, people like doing business with people they like even if they don't really know them. So choose your actions, words, and behavior with this in mind, and when you do your efforts will be rewarded, generally speaking. One of the biggest things that impacts relationship building in forums is familiarity. You have to be there and contribute on the forums regularly if you really want to build a serious presence.
In line with other comments about how you conduct yourself, or the attitude you create, you have to just be relaxed and cool in forums. You'll see all kinds of people if you're there long enough or over a long enough period of time. Some people will always come off as arrogant. You'll see folks who are habitually negative and complain a lot. Those types of people become ignored in forums and they will eventually stop visiting. These are the unwritten rules of forum marketing. So, you have to be concerned about how these rules will affect you and their importance. Learn how to follow the rules if you want to be successful.
You can think about forum marketing in next a similar way as social marketing. And it's no surprise that forum marketing is a form of social marketing. They are very closely connected to relationship marketing, which is why you should learn as much as you can about it.