Easiest way to do it - write it dead last.
Write your email in this order:
1. Body of email
2. Formatting: fix the paragraphs, bold/underline, create headers if longer piece
3. Subject line
Still i constantly receive mails that are not descriptive enough. If there are IDs to change requests, deploys etc, why don't you put that ID in the subject so it's easy to get to it later?!