Do no post the same message several times. If you think there was a problem with the message such that it did not get posted, wait a few minutes and check to see if your message shows up in rn. If you do not think your messages are getting to other sites, talk to the operators about it, rather than posting the same message again. Do not post copyrighted materials. Follow the rules of the group you are sending to.
Some groups have some rules of thumb to make life easier for the people reading the group.
Again, reading a group for awhile before posting is a good idea. Reply to other people's articles by mail unless there is a specific reason to do otherwise.