In a conversation, there is some minimum of shared context. You might be in the same physical location, and even on the phone you have, at minimum, commonality of time. When you generate a document for paper, usually there is some context embedded in the medium: the text is in the proceedings of a conference, written on a birthday card, handed to your professor with a batch of Econ 101 term papers, or something similar.

With email, you can't assume anything about a sender's location, time, frame of mind, profession, interests, or future value to you. This means, among other things, that you need to be very, very careful about giving your receivers some context. This section will give specific strategies for doing so.

Useful Subject Lines
A subject line that pertains clearly to the email body will help people mentally shift to the proper context before they read your message. The subject line should be brief (as many mailers will truncate long subject lines), does not need to be a complete sentence, and should give a clue to the contents of the message. For example:
Subject: need 3 thrombos by Tues

Chris - I need three thromblemeisters for Thursday's
demo in Boston. They need to be left-handed, and
they need to be packed for shipping by Tuesday night.

Here the subject line summarizes nicely the most important details of the message.
If your message is in response to another piece of email, your email software will probably preface the subject line with Re: or RE:. If your email composition software doesn't do this, it would be polite to put in RE: by hand.

Subject: Re: need 3 thrombos by Tues

Pat - I've got two thromblemeisters already packed
from last week's demo, but I don't have another
functional left-handed one right now. Can you
cope with two lefties and one rightie?

For time-critical messages, starting with URGENT: is a good idea (especially if you know the person gets a lot of email):

Subject: URGENT: need left-handed thrombo

I've *got* to have another left-handed thromblemeister
for the Boston demo, and I need it by tomorrow
afternoon. Chris only has two, and I've got to have
three. Chris *does* have a broken leftie, so if
anyone could fix that one, or if they have one in
their desk somewhere, I'd really appreciate it!

For requests, starting with REQ: can signal that action is needed:

Subject: REQ: turn in thrombos

Pat's call for a left-handed thromblemeister
turned up 12 functional lefties that were
lying around people's offices unused. Please
take a moment to look around your area for
thromblemeisters (rightie *or* leftie) that you
are no longer using, and get them back to Chris.

If you are offering non-urgent information that requires no response from the other person, prefacing the subject line with FYI: (For Your Information) is not a bad idea, as in

Subject: FYI: donuts in break room

The donut fairy left a dozen doughnuts in the
downstairs break room. First come, first served!

Information, Please
Do yourself a favor and eliminate the word "information" from your subject lines (and maybe from the body of your message as well). When I was the webmaster for the University of Illinois at Urbana-Champaign, I got a lot of email that looked like this:
Subject: information

Please send me information about UIUC.

This gave me very little clue as to what the person wanted to know about: admission application deadlines? The number of students? The acreage? The number of buildings? Was I supposed to send paper documents or give URLs? The only thing I could do with email like this was ask for further context. Mail like this would have been much better as
Subject: UIUC history

Are there any Web pages about the history of the U of I?

Quoting Documents
If you are referring to previous email, you should explicitly quote that document to provide context.
Instead of sending email that says:

yes

Say:
> Did you get all of the left-handed thromblemeisters
> that you needed?

yes

The greater-than sign (>) is the most conventional way to quote someone else's email words, but your email software may use a different convention.
Even if there are a fair number of words in your response, you still might need to quote the previous message. Imagine getting a response on Monday to some email that you can't quite remember sending on Friday.

I talked to them about it the other day, and they want to see
the other one before they make up their minds.

Your response would probably be the highly articulate, "Huh???" It would be much easier for you to understand email that said:
> I've got the price quote for the Cobra subassembly
> ready; as soon as I get a decision on the
> thromblemeister selection, I'll be ready to go.
> Have you talked to the thermo guys about whether
> they are ready to go with the left-handed thrombo or
> do they want to wait and check out the right-handed
> one first?

I talked to them about it the other day, and they want to see
the other one before they make up their minds.

This is substantially better, but now errs on the side of too much context. The first three lines have nothing to do with the question being answered. You should only include enough to provide a context for the message and no more. (Peter Kimble, my high school computer science teacher, now gives his students the heuristic that at least half of the lines in an email message should be their own.)
You need only enough context to frame the question being answered:

> Have you talked to the thermo guys about whether
> they are ready to go with the left-handed thrombo or
> do they want to wait and check out the right-handed
> one first?

I talked to them about it the other day, and they want to see
the other one before they make up their minds.

Remove Pronouns
The above example gives a good amount of context, but the response to it still takes a little effort to follow. A good rule is to look very carefully at all pronouns in your first three sentences. If they don't refer to something explicitly stated in the email, change them to something concrete.
> Have you talked to the thermo guys [about which handedness
> they want]?

I talked to the thermo group on Wednesday, and they
think the left-handed thromblemeister will probably
work, but they want to evaluate the right-handed unit
before they make up their minds.

Now the answer is very clear and specific. And, since the response contains implicit yet clear references to the original message, less explicitly quoted material is needed. Responses like this, with the context mostly in the body of the message, are the easiest to understand. Unfortunately, they take the longest to compose.
If you want to quote a sentence that is in the middle of a paragraph, or wraps around lines, go ahead and remove everything but the part that you were really interested in, inserting "[...]" if you have to take something out in the middle. You can also paraphrase by using square brackets, as above.

If the message isn't important enough to you to warrant the time to pare the original message down, include the whole thing after your response, not before. If you put the original message at the end, your readers don't have to look at it unless they don't understand the context of your response.

Summary
You may know what you are talking about, but your readers may not. Give them the proper context by:
Giving useful subject lines
Avoiding pronouns in the first three lines
Quoting the previous message