Ask HN: what have you learned in the last 12 months that help you make money?

Last week I started wondering what I had learned over the last 12 months. What did I learn that actually make a difference on my bank account? Of course I learned many small things (tricks in PHP, javascript, css, http server optimization, etc.) But these things didn't really make a difference. Still there were a couple of things that truly were "breakthroughs" for me, even though there might seem trivial to many.

Here are two things I learned in the last 12 months that "moved the needle" for me because they both helped me improve my conversion rate to a point where I can see the possibility of a positive Adwords ROI:

-> Adwords / I can track conversion by keywords, and associate values to conversions depending on user input

-> Website speed / Cloudflare has a free CDN that works well (using the full performance optimization gives me a 95 score on Chrome Page speed, and Google Analytics seem to report page load under 2 seconds - although it is a bit early to tell for sure)

What breakthroughs have you made in the last 12 months? What significant lesson have you learned?