Keep your messages short to conserve bandwidth and save the resources of those subscribers who must pay to receive messages. Some gateways also limit the size of mail they accept. If your message is longer than about 20K, you might consider posting a short abstract and invite interested readers to request the complete version from you. If you have a longer posting of general interest, contact the List Owner to see if it can be archived on the file server so people can request it. Use a descriptive subject line.
Many people decide whether they want to read a message based on the content of the subject line.
If you want to start up a conversation with someone else, send it as email directly to them, not to the entire list readership. Sign all your messages. It helps to have at least a name to associate with a post and some mailers strip off the header, so the receiver cannot reply unless there is some kind of internal address. As Internet marketers we would strongly recommend that you develop an appropriate signature tag indicating your email address and homepage if any at least. Remember, somebody may need your services one day and you are making it easier for them to assess you if they have some access to information on you.