When you hit "Reply", most programs include a quoted copy of the original message. Delete as much of this as you can, while still making your message make sense. The other person often knows what they said -- they don't need to relive it.
This is especially important when the email has many diffent replies to it. Just keep the pertinent stuff.
This is similar to the "Trim Quoted Text" above. When you forward somebody something, delete all the extra headers that appear before the actual content of the message. Who wants to scroll through every other person's email address who has been forwarded this email in years past.
Clip out everything but the real content. It'll make your dumb forward much more likely to be read. :)
A common newbie thing is not to change the subject line when you send someone an email. Admit it. It's much easier to pick a message someone else sent you and just hit "Reply" to create a new message for them.
You type your message about how well your summer vacation went, then hit "Send" and off it goes. With the subject line of "Re: Christmas Wishes".
Especially if you're sending a message a person may keep in their Inbox for a while, do them a favor and change your subject to reflect the topic of your email. It will also help you if you ever have to do a search for this message in your "Sent Items" or "Sent Messages" folder.
Tip: If the email you are sending asks the person to do something or poses a question, consider putting that in the subject line. Then, when they're looking thru their emails, your task request is right there in the summary. This tip is especially handy for use at work.