When responding to a post, a pretty common listserv courtesy is to leave just enough of the original message intact so that someone jumping into the thread will know what your message is responding to. This is called "trimming" your posts.

A while back Macromedia's John Dowdell wrote an excellent article on using email discussion lists and forums effectively. It should be required reading for anyone who participates in a listserv or online forum: http://www.macromedia.com/desdev/jd_forum/jd014.html

And here's a good basic list of listserv etiquette: http://www.bccls.org/buckles/listservs/LISTSERV_ETIQUETTE.html