AttachmentsAlways a bad idea (*especially* binary ones). Text "attachments" should simply be incorporated into the body of the message.
QuotingDelete each line to which you are not replying. 40 lines of quote followed by two lines of reply will not be read by over 90% of us. We need to see the substance of your reply in the first 10 lines.
Subject linesThe archives is capable of organizing the "threads" of discussion. For this to succeed it is essential that the subject line be changed (preferably with a (Was Re: old subject). Those of us receiving the digest version are strongly encouraged to send separate messages in reply to each item and to alter the subject line to match the "thread".
E-mail standardsDespite what the international media and others have to say about the World Wide Web and rapid technical change, email works on a much older set of standards. For a variety of reasons many us can only participate 'because' of those older standards. Don't send HTML style mail (the single exception being individual accented and other characters that are a necessary part of communication in a variety of languages).