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. That may sound like a very long time, and if so then maybe you believe all the sales letters. But if you want to get maximum value from your efforts, it will require some time and patience on your part. When it's your business on the line, you'll have to decide how to proceed.
If you want to be viewed as an expert in your niche, you must begin by working to create a good reputation. That's the only way to place yourself in a position for prime results. But you can't really get noticed as an authority without first being seen. But you're not interested in having your entire day spent on forums. After all, there are other things you need to do to market your business too.
It will also annoy regular members if you come on strong and make a ton of posts in a matter of hours. Find out which hours see the greatest amount of activity and make your forum posts when the largest number of people are generally present. A small number of thoughtful, well-stated posts will do much more in this busy timeframe than a hundred small posts made just to get your name out there.
Remember that it's always wise to contribute by starting your own threads and conversations. Starting your own threads may be a little difficult at first due to shyness. Begin by thinking of information you have that can be valuable to the community and share it. Don't forget to encourage others to ask you any questions they might have about what you've said.
If you really want to invite commentary, try saying something that's a little controversial. You may have to walk a fine line when it comes to controversy and it can be a judgment call. Don't go outside the niche when you start threads, whatever else you do.
If you really want to go all out with your networking efforts, then find out who has influence in the forum. Sometimes blog here there is a group of people that could be at the 'top of the heap' on the forum. The number of active members in a forum will have some influence on this too. The longer the forums have been around and more members it has, the greater the number of people who influence the forums will be. The influencing figures are different on different forums. With senior members and moderators you'll often be able to recognize others attempting to garner favor. These are the people you want to get to know as quickly as possible on the forum. Even though forum marketing works, do not expect immediate results. However, you could score a few quick sales if you find the right forum and product. Some forums allow you to advertise your products or services, and when done right that can bring a fast cash windfall.