How to make your clients open mailings?

CMS Producer - our content management system has a feature which allows you to easily prepare a direct mailing campaign to your customers. The question is: how to make your customers open the mailing?
Remember 3 important rules about direct mailing:
Golden rule #1: Added value
Each mailing should include the added value for the customer. Not always it has to be a typical material value but let it be something that you prepare specifically for your client, for example, a report prepared exclusively for a particular industry, an article which helps to market their services better, or perhaps an interesting example of a marketing campaign (see below).
Golden rule #2: Highlight subscription benefits
Customers who subscribe to your newsletter is a unique group. Prepare something special for them, "prize", which should be given only to them because they are loyal to your brand and read your newsletters. For example, "2-minute course of telekinesis only for subscribers" ;-) (see below).
Golden Rule #3: Interest is the key
If you show your customers something they have already seen their curiosity levels begin to fall and consequently your mailing eventually ends up in trash. How to spark your customer curiosity? See for yourself - no one has ever seen it before.