At the outset let me emphasize that your email / mail drafting skills is a subset of good communication skills. I would therefore stress that one must focus primarily on effective communication skills. Email is just one popular and extensively used medium of communication which may have some peculiarities and requirements which can be worked on separately. So let me first share some important aspects of effective communication skills followed by some pointers on drafting result oriented emails.
In my view the most important element in any form of communication is the clear understanding of what, how, and why you are communicating. This can be clearly understood by ensuring that you write the subject line of your communication even before you begin writing anything else and also proof read for typographical errors.
Next ensure you follow the rules of good drafting, editing and proof reading. These could include one idea per para, flow of thoughts, use of punctuations, following the 5'c of effective communication etc. Ideally I recommend that all communication be typed in WORD, thereafter proof read and then as required cut and paste.
As a habit/ practice, ensure that for all forms of communication including casual communication, the content is well drafted, the sentences and paragraphs are constructed using all the rules of grammar and effective drafting skills. You would notice that more and more people are using the SMS language, using short forms, jargon, liberally tossing out the rules of grammar when they post matter in this site. If one gets into this routine, the quality of formal communication would suffer in the long run and this would be evident in the letters, reports, emails, inter office communications, circulars etc. that one generates.
You may also note that one must be more positive, proactive and prompt in your communication. This can come about if your own attitude, behavior and interactions always reflect these traits. Try to minimize the use of negative words like no, can't, difficult, etc. even in every day communication.
When it comes to emails the first rule is to keep the communication short, crisp and to the point. Avoid use of adjectives as the communication is intended to be informational or focused on getting some action. Keep in check the temptation to mark copies to one and all; instead limit it to only those actually concerned. Revert to incoming emails at the earliest. If a phone call can get the work done faster, speak to the concerned person and only if required summarize the talk in writing through an email. Needless to say, being polite, courtesy and well mannered in style and content would be the icing on any form of communication including emails.
I would also invite you to visit the following Inspirational and motivational Blogs which could also give you a flavor of effective drafting skills
7th June 2013 From India, Mumbai