There are several tips there, but here's the summary:
- Write from your college or university e-mail account.
- Include the course number in your subject line.
- Choose an appropriate greeting.
- Avoid rote apologies for missing class.
- Ask politely.
- Proofread what you've written.
- Don't send unexpected attachments.
- When you get a reply, say thanks.
I'd only add one more to Leddy's suggestions: For many instructors, their e-mail inbox is their main electronic workspace, and it's often very crowded. Your instructor's time and workspace is a scarce commodity, so respect it. In particular: Don't send your instructor e-mails with questions that are already addressed on your course syllabus.
Other suggestions for students as to how to compose an e-mail to faculty?