So, don't post blatant ads. Also, don't post off topic comments. Make useful comments. Of course, most good discussion lists are moderated, meaning that a moderator views each message before approving it for distribution to the list. Still, you should be careful in what you send.
Moderators are usually very busy people.
When you have seen what others post, and have received a general idea of what is acceptable, you are ready to make a post. Wait for someone to raise a question. If you can, answer it. As I have said before, you will become known as an expert in your field if you give good answers to people's problems. That's a good thing.