Good subject headers are short and specific. Put the most important message information early in the message, and use good writing techniques including subheads and short paragraphs. That's because most email readers won't scroll down to see more of a message unless they perceive a compelling reason to do so. Give them the reason in the early part of the message. Be succinct and conversational.
Email is more like conversation than it is like formal mail Short and sweet works best.
When replying to messages quote portions of the message you're replying to as part of your reply. If you've chosen good email software, this should be automatic. Use filters on incoming email to make your mail handling easier and more effective. Set up multiple mailboxes for different types of messages. Then use filters to automatically sort messages into the appropriate mailbox.