Forum is the most effective and fastest way of selling affiliate product. It may be by direct linking or list building. But what works for Ptc Advertising won't work for a different type of linking like forum.
1) Forum is just unique form of getting traffic and if you want to make direct sells, I recommend that you must invest your time by reading the terms and conditions. If they allow linking, yes you may go ahead. But if they don't allow ads at all, leave it.
2) Begin by building reputation.
Be very quick in responding to a new thread.
3) Give an important comment, increase your popularity by broadling the discussion.
4) Be an authority to what you say and never ask too many question on a forum board - you are the one who supposed to be answering questions.
5) Be as someone who is full of knowledge on what he's discussing. This will make some desperate people to even ask you a direct question.
6) Avoid meaningless posts and flamming.
7) Contribute only to busy forum that recieves much traffic.
8) Once you have developed some quick reputations within a week or two, you can begin advertising with your forum signature.
10) Don't forget to use colours and some eye-catching assortments. This will draw attentions as to many traffic.
11) With this signature, you may direct the link to an autoresponder email capture or to the affiliate sells page. You may try both, in order to choose the best out of them.
12) Remember to do this processes in many applicable forums especially those with; high traffic forums, allow commercial links and has to correspond directly to your exact affiliate marketing Goals.