I will start the ball rolling.
Every { and } goes on its own line with no other code.
Code to be executed when an if is true always has { and } around it, even if it is only one line of code.
Give variables appropriate names to indicate their purpose.
Use defines or variables to give pin numbers meaningful names.
Consider putting blocks of code in appropriately named functions to make it easier to test and debug.
Use Auto Format often. Not only does it tidy up the code it points out missing/extra brackets and braces.
Test small sections of code or functions as you go along using appropriate test data, hard coded into variables if necessary.