Trust and credibility
Whatever you do never make your e-mail promotions looks like spam and above everything else never send out spam!
Only use $$$$ and CAPITAL LETTERS when appropriate and don't overuse it. We all know that spam look like this often delete these mails without reading them.
Bad spelling and incorrect grammar is a credibility killer so make a final spell check before you hit the send button.
Make it clear and simple to understand how to "opt-out" from your newsletter. Readers are more likely to "opt-in" if they know they can easily "opt-out" if they don't like it.
Make it interesting and fun
Write in a personal and conversational style but still keep it professional.
You can make a few jokes but be extremely careful because jokes can easily be misunderstood. Be very careful not to offend minorities or other groups of people.
Make it valuable
Make sure your readers find something valuable to read in every e-mail you send out. Your readers are drowned in so much information that they can't afford spending time reading useless junk. They will immediately look for the opt-out link if you do.
Make it simple
Write in a simple language but not so simple that you are "talking down" to the readers. Remember that it's not a complicated technical report you're writing.
Break up your text in short paragraphs of 1-4 sentences. Restrict each paragraph to one topic and use a one sentence paragraph to emphasize something important.
Have your objectives clear
Decide what you want to achieve before you start writing. Do you want to sell a specific product or do you just want more people to sign-up to your list. Whatever it is you should decide your objectives in advance.
Your objectives should be aligned with the readers interests.
I suggest you are upfront with what you want to say already in your subject field, headline and first paragraph.
If your readers are interested they will continue reading and hopefully find something of value. If they don't find it interesting they will quickly stop reading but that's OK, at least you haven't wasted their time reading "junk".
Make it unique
As an affiliate it's tempting to use the standard material you get from the merchants. The material can be great but I however suggest that you customize it to make it unique and with your own style and "voice".
Imagine a reader that get a three newsletters from three different sources and they are all identical.
It doesn't project a good impression!
Make it regular
Commit yourself to a regular schedule for the news letters but don't make it too regular. Too much can make it look like spam.
Make it viral
Encourage your readers to forward your newsletter to their friends. This is more likely to happen if your newsletter is interesting and valuable for the reader.