Forums are great places to find other like minded entrepreneurs who can help you with your business. You can also help them. Remember, networking is a two way street. While you are learning and teaching useful information you can also get the word out among your peers about your business website. 
One advantage of niche forums is the connections. You will meet people who have made it big. Here you are all business people and you don't feel small when you ask questions. The environment is one of camaraderie and is supercharged with money-making energy.
So, what's the key to promoting yourself here? One thing you probably already know, but bears repeating are that forum communities discourage blatant advertising. Besides, you won't make any friends or useful contacts that way. It is thought to be a privilege to belong to professional organizations so using them the correct way is important.
Usually, you will be allowed to mention your website in your signature line. Here, you can feature your link and a catchy tagline if you have one.
So, what's the key to being seen in these forums? In a word, it's getting involved in the conversations. Find threads that interest you and follow them. When you feel like you have something to contribute join in and post a comment.
People get to know you when they see you around the forums regularly. You can begin your own threads and talk in existing ones. Find your way around the different areas of the forum so that others can see that you are an active member and not just there to talk about your website. Other business owners can easily spot someone who is around for nothing but their own personal gain. They will respond (or should I say NOT respond) to a member they feel is doing so.
There are ways to get your website mentioned in posts but only when it is appropriate. When you have pertinent information to offer to others and it happens to be found in one of your blog posts, linking to it may be acceptable to forum moderators. Make certain that you ask first if posting additional links are okay in relevant posts. Remember, you want to be helpful not annoying.
Another benefit to interacting with a group of professionals is that you can find others with large followings. Building a relationship with them can lead to their endorsement of your website and possible quality backlinks in the future. Those same people may even be interested in entering into joint ventures with you in the future.
If the forum has a place for posting what's new with your business, then by all means, talk about yourself and what you do there. Even with permission, keep it modest and read about the information of others and comment on their posts; visit their sites as well.
You can promote your website in professional forums without wearing out your welcome. All you need to do is participate fairly and help where you can. The advertising will come on its own once you do.
