Being active on forums and mailing lists can help bring traffic to your website. On some forums you can place a link to your website(s) at the bottom of every message you post (your signature). These links can help increase the number and quality of your incoming links. They can also help you gain visitors who click on the links in your signature.
Forums and mailing lists can also help you keep up to date with the latest news in your businesses field of expertise. Actively answering questions on forums can help establish your reputation as an expert in your field. My suggestion about participating in forums and mailing lists is to stick with mailing lists and forums that you would participate in even if you didn’t have a business to promote. The reason that I say this is that you can spend a lot of time every day on forums and mailing lists keeping up with the posts. This is time that could be spent promoting your business in other ways, running your day to day business, or having fun with your family.
Participating in forums, discussion groups, mailing lists, etc. helps you become part of the community for the niche you are in. It is ideal for you to be participating in forums or mailing lists in your niche a long time prior to starting your business. Because one of the best ways to get instant traffic to your site is to already be active in the community your site is targeting.
While forums can help you get traffic, they can be very time consuming. You need to balance your forum participation with your goals. If one of your goals is to work smarter and not harder, then I suggest spending a minimal amount of time on forums and spend more time using other promotion methods. That said, forums that allow you to place your website in your signature certainly can help increase the traffic to your website and forums can be a great way to get involved with the community you are targeting.
One important final note: It is better to be active on a forum with a huge membership than on a forum with very few members. It will give you much more exposure in your niche.