Posting on forums is one of the cheapest and fastest ways in which you can advertise your affiliate products on the Internet. However, in order to do this successfully, you must do a number of things with care. You cannot simply start a thread which contains your affiliate link straight away. You will almost definitely get banned from that forum board and you certainly won't get any sales from it.
The general procedure involves making multiple posts on a forum board before you even setup a signature file, which includes a link to your site. Even if the particular forum you are using does not specifically mandate this, you will want to do this, anyway, as it will increase your credibility - which is your real goal.
Finding a forum related to your niche is relatively easy to do. Simply type the name of your interest followed by the word 'forum' into your favorite search engine i.e. if your niche is golf, type in 'golf forum' into your browser and you will likely get a list of active forums on the return page. Alternatively there are a couple of URLs that you can use to obtain the same information. They are" big-boards.com" which is currently the largest forum directory on the Internet or you can use "board123.com/forum_directory.php" which has some lesser directories but does contain some forums that the first doesn't list.
Once you have chosen a few forums that you are interested in, you will need to find out what their rules are when it comes to signature files. Some don't allow links and others do but may not accept these links to be of a commercial nature. It's vitally important to stick to the rules of these forums or you will find yourself banned pretty quickly.