What are the most valuable lessons that you have learned whilst working in procurement?

Being new to procurement, I am eager to know valuable lessons that others have learned during their time spent working in procurement and contracting. An associate wrote an article discussing his '5 Takeaways' from his time in procurement and they have assisted me in both my work and personal life. Having proudly negotiated a large discount from my broadband and mobile provider. But what other lessons, tactics or strategies that can be easily adopted in procurement? Here is the blog that I mentioned: http://blog.marketdojo.com/2016/08/guest-blog-five-takeaways-from-20-years.html