Here is one that works so well for me: "Never change more than one thing at a time".
This applies when you are trying to sort out a bike fit issue or component set-up, etc. As someone that tries to read the tea leaves of bike performance as a test/reviewer person, I will sometimes need to tweak things here and there to see what is causing an issue.
A typical error would be to raise your handlebar height AND change the saddle height too. Or slide your saddle back a bit and change to an alt bar with lots of sweep. Now your back hurts. Which is the problem? Aaacckkk! One thing at a time please.
How about a new front tire and a shorter stem? Now you are finding you are washing out on corners that were not an issue before. The tire maybe? Or perhaps the stem has moved your weight too far back now? See?
Keep it simple and keep it singular.