The Test Your E-mail I.Q. post at the Grammarphobia blog offers 20 tips to consider before clicking "send"; here are some of the best:
1. Is the subject line helpful?
The reader should be able to tell at a glance what you're writing about and how urgent it is.
2. Did you get right to the point?
4. Did you say too much?
Try not to write more than you have to, especially at work. Long e-mails get read last.
6. Did you say what you're replying to?
A cryptic "Fine" or "Nope" or "Maybe later" won't do. Never make the reader guess which message you're answering, especially if time has passed.
12. Did you break for paragraphs?
Please, no solid blobs of type. Break your message into bite-size pieces, one short paragraph per subject. It'll be easier to read and easier to answer.
18. Does it have to be an e-mail?
Maybe a letter or a phone call or a face-to-face meeting would be better.