Snip! Snip!

DO cut (snip) mercilessly.: Leave just enough to indicate what you're responding to - replacing the missing text with the term '<snip>' or similar. Never include headers except maybe the "From:" line. If you can't figure out how to delete lines in your news software, paraphrase or type the quoted material in.
DON'T include the entire contents of a previous posting in your reply.: For a start, some people still pay to download posts. They'd rather not pay to read old material all over again.
Secondly, screen after screen of old, quoted material doesn't exactly enhance anyone's reading pleasure - especially if your contribution is limited to just a few words/lines at the end. If the amount of quoted material exceeds the amount of new material in your response, STOP and see if you can apply some editorial shears first.
DON'T post an OLF (One Line Followup) unless you are quite sure it will be worth someone's time and effort to read.: See above…
DON'T remove the attribution lines for any quoted material you are retaining: It is only polite to ensure that, if you quote someone else's words, you attribute them correctly. It also makes it quite clear whose comments you are responding to. Also, bear in mind that a dozen articles may occur between the original post and your followup. For some people, your reply may even get there before the original!