Who is your audience? What is your target market or niche? If you're targeting a specific niche like home security then perhaps you only need to sign up to ADT and SpyBase so your products match your audience. There’s no point promoting eco-mattresses to your blog that’s focused on reviewing drones. Again, you could sign up to a network that has a few different options in your field of interest or just go straight to your favourite suppliers and see if they have an affiliate program.
Great post , I do read a lot of the Nichehacks articles and this one is so true. At the moment I am in a niche I'm passionate about and yes although I am primarily using Amazon to monetize my site, I will be branching out to use other methods very soon. It frightens me to think the plug can be pulled at any time! I intent to use other affiliate programs as well as Amazon, maybe Google Adsense, I'm not sure yet, some digital products and also to build an email list.
How established is your site/blog/social media following? Do you already have an audience of 1 million or are you just getting started? Early on in your affiliate marketing journey you will probably want to have multiple options to make sure you can cover a broad field. You might want to become an Amazon Associate as well as joining one of the networks so you will have a product link to drop into any posts you do. An easy place to start is to promote the services you are using if you are enjoying their product - web hosting or email management systems like Aweber.