If I had to pick one thing I wish marketers did more often, it would be using forums to do market research. Whether as a lurker or a sometimes participant, keeping an eye on what your target market’s actual thoughts, opinions, and desires are is a best practice you can’t ignore.
See? There's a picture and everything. Who's going to argue with Aragorn?
Getting The Research Done
Sure, you could pay to have a survey conducted. Or you could go to the source for more accurate and detailed information.
Users generally don’t feel like they’re being watched and have no expectations, so they’re (brutally) honest in their thoughts and opinions
You can figure out what the other guy is doing right
You can get a sense for the kind of culture and lingo that surrounds your product
You can refine your product to meet their needs
Using this information you can improve your brand quality and create highly targeted ads that will give an insider-approach appeal to your customers. Showing that you're in touch is a big part of the game. Which brings us to the next point.
Engaging Your Customers
Lurking is dandy, but engage through popular forums and you'll win the hearts of the public. Just take a moment to hang out. Read. Relax. Give product updates, respond to concerns, let people know you're listening and that you care. Talk like a person, not as a PR bot--that means casual language and expressions.
When reps of a company respond to criticism or answer questions online--and not on their own website, but actually by taking the time to seek out their community--they become highly respected. Assuming, of course, they're being helpful and not representing the company in an unreasonable way. Then you're a few hot-headed replies away from a viral PR disaster.
The Method in Action
I'll go back to the Reddit thing, because it's a great starting point for people looking to find discussion groups and see examples of other companies properly engaging their customers. Which is what Reddit is for: engaging, not marketing. This is anecdotal, but you'll see my point from this example.
When I'm not hanging around and answering questions in /r/marketing, I'm often in the wet shaving subreddit /r/wicked_edge or /r/cigars, because I'm a manly man marketing machine. (For those unaware, wet shaving is the traditional straight or double edged razor and brush method, barber shop style.)
Both of these subreddits have reps from some of their favorite brands who pop in from time to time to answer questions and respond to concerns. A new cigar manufacturer, or a specialty shaving soap maker, might send a free sample here or there, or let loose an important product announcement (Conversational! Not press release style!) and as a result they gather a lot of attention. This can lead to a significant boost in online sales for smaller companies looking to get discovered. Since users of one forum are often users of multiple, it's easy for the content you post to catch like wildfire. I'll often find something posted in /r/wicked_edge being discussed in the forum Badger and Blade, or vice versa.
This is the most effective way of word of mouth marketing online. If you create champions in message boards, they'll carry your brand far and wide. All you have to do is be accessible and have a good product, and you'll quickly gain a reputation as a thought leader.