General advice for WLAN with multiple Access Points?

Inspired by this excellent thread, I did a few searches on the forum for general advice about setting up a network with several AP's, but all the advice seems to be dispersed in problem-solving threads.

What best practices should I follow choosing, placing, and configuring Access Points? What mistakes to avoid?

Although this video focuses on the unifi products, it is a good explainer for how to optimize channels, power, locations, and other factors for WiFi. The concepts are pretty universal, but the specifics about how to implement are obviously going to be a bit different on unifi vs OpenWrt vs other platforms.