We work with a lot of affiliates, but our rules are a bit different. In order to be an affiliate for WPML or Toolset, you need to be a client and to actually use our plugins.

From the start, I didn't like the idea of people writing about our plugins without even knowing how they work. Our affiliates use our plugins and recommend them from their experience.

So, we rarely appear on 'Best 15 plugins for XYZ'. Our affiliates normally write content that adds value, using their expertise or unique experience.

Typical great affiliate content:
* How to run a multilingual site with theme NNN and WPML
* How to use Views plugin to build NNN
* A course/tutorial/training on PLUGIN

We're also against coupons:
http://wp-types.com/2014/11/black-friday-cyber-monday-sorry-playing/

An affiliate that promotes our plugins will not be offering a 'unique discount for my readers', but unique content and information.

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I agree with Nate. This takes a lot longer to get off the ground. For our Toolset plugins, it took 3 years to break even. But I think that it's more sustainable.