Let's assume that you have a big list of emails. Probably you've build it in order to use it for selling something, and even if you haven't - you should do this.

There's a big potential and a lot of possibilites in maintaining mailing lists.

So... the question is how to write a letter that really sells?

Here's a bunch of "technical" advices:

1. use plain text instead of html -

a lot of people don't read html letters, use older or simpler mail applications, sometimes html e-mails are blocked by antivirus software...

2. never use images as main content -

it slows downloading and also can be blocked by some mail clients

3. never use line longer than 50 characters -

this way the text will be always visible on the screen and will be easier to read

4. make message clear visually -

your letter may be quite long, but always use headers, outlined questions, use paragraphs and interlines. This will make you message easier to read and understand. Remember that most of people do not read but scan the text

5. do not sell, instead give solution to the problems -

yes, that's very important. If you try to sell, people will not buy. People just want to make own decisions so if you identify the problem (or make them thinking they experience one...) give the solution. This will make people think "oh, finally i've found the solution, let's give it a try...", which will follow in a straight line to the transaction!

6. do not provide long, complicated links -

such links can be broken in mail clients and also a lot of people simply don't trust "unreadable" links. If it's your affiliate link you may also loose your earning when some people just cut out your affiliate id from link.
Here is solution to this one: make short cloaked url.

Hope this will help you.