Be clear in your own mind on the purpose of your message: to deliver a piece of information, to report on status, to offer an opinion, to persuade, to make a request, whatever. Focus on what serves that purpose and let the rest go.
Cover only one major topic in a given message, and make sure the subject line reflects that. Save other topics for other messages.
Avoid explaining anything that isn’t essential to an understanding of the message.
Offer further details on request rather than supplying them all.
Don’t indulge in self-justification.
Don’t show off.
Don’t say anything that needs to be said in parentheses.
Avoid complex sentence structure as much as you can while still making your point.
Break up long paragraphs. Use bullets where you can.
Don’t serve a hidden agenda unless you can do it with careful word choice as opposed to added verbiage.
Writing concisely does take longer if you’re naturally verbose. But you can probably reduce your word count by at least one-third without significant loss. The payoff is a message that gets read rather than just skimmed or ignored and trashed.