I was in a technical conference a while back and attended a session on effective/professional e-mail communication. It wasn't something I'd really thought of before, and the session included some good tips, some more obvious than others. A partial list (what I can remember) in no particular order:
1. Add the addressee(s) last so you don't accidentally send the e-mail before you're ready.
2. Before doing (1), re-read the entire e-mail and make sure it makes sense/doesn't have any glaring typos. And attach the attachments! (I'm forever sending follow-up e-mails with attachments).
3. You have a subject line for a reason. Use it. The recipient should be able to tell by the subject line what the project is and what...well, what the subject is. This leads to the next point.
4. Sometimes we end up with long e-mail chains. That's fine, but if you end up changing the subject over the course of the e-mail conversation or you're following up on one particular bit, it's ok to start a new e-mail with an appropriate subject so that months later, you can find the relevant message at a glance.
5. You don't need to have the last word in e-mails. If someone writes "thanks for the response!", you don't need to respond with "you're welcome!". Especially if you're replying to everyone. This isn't a phone call. Stop filling up everyone's in-boxes!
6. E-mail is a quick communication. Most people will spend about 10 seconds scanning each e-mail. So if you're going to mention several different things, use bullets or numbering, or clearly break up each separate item into a (short!) paragraph.
7. To go along with #6, some e-mails are FYI and some are "this requires your action". Make it clear at the beginning what action is needed, if any.
8. I have a project-based system for organizing my e-mails. You can organize the buggers however you wish. But if you're like me and you coordinate with a few people on several different projects at a time, it will make your life infinitely easier a few months later to have them organized by something other than what you get by reshuffling e-mails by sender name and guessing at an approximate date.
9. This apparently is a big no-no, but I do use my "deleted" folder as a storage device. It's the same thing I use for the recycling box under my desk - stuff I don't think I'll need again but that I'm not willing to permanently remove. Every once in a while, when my e-mail gets too full or the pile in the box looks like it's about to crawl out and run away, I go through and get rid of the old stuff that I couldn't possibly need anymore.
10. Don't put anything especially snarky or obnoxious regarding a client (or anyone else you work with, for that matter) in an e-mail, and get rid of sarcastic e-mails that others send you. You write something rude as a commentary on an otherwise factual e-mail, the recipient snickers but then follows up on the technical comment, the obnoxious comment gets buried in the back and forth, the e-mail chain gets sent along to a 3rd party by accident, and then you're in for a world full of hurt.
Anything else you can think of?