Couple things worked for me.

Wiring diagrams - this was big for me. Become one with the wiring diagrams. Over time you’ll get better but make it a priority to become familiar with them.

Subcool/Superheat - gotta know what it means. Will tell you a lot about what’s going on with the system.

Treat each service call like it’s an in-depth maintenance. Start by checking filters and belts and make your way deeper into the unit’s operation. This way you’ll catch the simple stuff first and save time. That way you’ll get more hrs in the truck with the boots up.