No matter what the communication piece (a 140-character-long tweet or a newsletter), or the context (affiliate-oriented, customer- or client-focused, etc), it will benefit from the following 3 Ps.
In fact, I believe these 3 elements to be integral components of any effective communication.
Let me illustrate each by looking at things in the context of affiliate-oriented newsletters sent out by online merchants.
1) Personalization — Use all available means (many merchants do not use the short codes that affiliate networks offer to them for personalization of emails to affiliates) to make it a personal and personally-relevant communication piece
2) Preciseness — Flee from vagueness, and be concrete (e.g.: not “our affiliates enjoy some of the best conversion rates in the industry” but “during the month August our affiliates’ average conversion rate was 2.3%, while our top 3 performers have enjoyed 0.8%, 2.45% and 5.71% conversion respectively”)
3) Practicality — This one flows right out the previous point. Examples of practicality would include lists of bestsellers, best-performing links, affiliate tools, new coupons, etc. It is also important to mention that including the information on these in your affiliate newsletter is not enough. You want to also give your affiliates an easy way to act based on this info. For instance, just yesterday I received an affiliate newsletter where a merchant is listing their best-performing links, but doing it in an extremely inconvenient manner. Instead of including the code for each link in plain text (or a separate box), they linked text within the newsletter through my affiliate URLs. So, to grab the actual code for each link mentioned I have to view the HTML code of their newsletter as opposed to a simply cutting-and-pasting. So, make sure your practical advice is also convenient to implement.
The above points will also work well in contexts other than affiliate program management.