Social Proof: What It Is and How to Use It (with Examples)

We’re all familiar with the standard “best practices” of CRO. Always use social proof, always reduce form fields, never use image sliders, and so on.
As someone who believes that best practices are merely common practices, I’m always looking to test the tried and true to see how, well, true it really is.
First up? Social proof. Does it really work as well as we all assume? Why? And more importantly, what’s the best way to implement it?