The BEST advertising. . .
In a forum such as this is active participation. It doesn't have to be constant, but checking in every so often and participating when appropriate with your specific knowledge can be a real help. Of course you need genuine interest in the forum's focus to be effective.
I know, in a popular international photography based forum, a well known executive From B&H Photo (A major N.Y. based photo shop with a strong mail order and Internet marketing business) is a regular contributor. It isn't frequent or excessive, but when the topic is related to marketing or buying reliable equipment, he makes a valuable and reliable resource without beating the drum for B&H.
I can't begin to tell you how much it has helped B&H the business, because I don't know; but I do know B&H has a VERY high reputation even with participants who live outside North America. The New York B&H Store is an important tourist destination for people who care about photography.
I know my last 2 photo equipment purchases were influenced by him.
BTW, you'd be amazed how often covers come up on this forum.