Many affiliate programs nowadays provide you with promotional materials that you can use. These sometimes cover the full spectrum of advertising, such as banner ads, PPC ads, classifieds, text links, ezine recommendations, solo ads, forum signatures, and even blog posts!
Try taking the provided material and see if it is appropriate for your list.
You can always rewrite what was provided in your own words and using your own usual tone of voice. As most of your subscribers know how you normally write to them. If you use the text provided by the affiliate merchant verbatim, chances are that they will not know that it was written by you.
Also, many of your subscribers are also subscribed to other online marketers’ lists and they will be receiving the same promotion from them. If your email looks exactly the same or even similar, your subscribers are going to lose some respect for you, as you have just regurgitated an email supplied by the affiliate merchant.
It is important that you provide your own perspective on why they should purchase the affiliate product in question. One of the strongest perspectives that you can provide is that of someone (you) who has purchased the product and is giving a review and letting them know how it has helped you, and how you are using the product in your everyday life.
If you can create or offer a unique and special bonus that ties in and compliments the product, then make sure you do that. This bonus could be as simple as a tutorial, a field book, or a companion guide. It can be in the form of an ebook, audio or video (so long as it is digitally delivered). This will make your email, ad or promotion stand out and avoid the problem of being just another "me too" ad which your subscribe will be tired of getting.