This is the real key to successful newsgroup marketing. There are thousands of people out there shouting their wares on every 'streetcorner'. These people are wasting time and bandwidth. The giving of yourself in the form of real advice or solid information builds your reputation as one who knows their subject. Repetition of this giving of yourself in these related forums will build an awareness, a presence for you that will bring customers to your 'door', whether it be a website or an autoresponder address.
Repetition is key to building a presence in newsgroups.
Stopping in once in a while to see if there is something you could offer to someone just to get your message out will not be nearly as effective as becoming a regular participant in these forums. Be there every day. Be involved every day. Give of yourself.