1. Your investment in torque wrenchs should be twice as much or more as your investment in impact wrenchs. I was told this by a wise technician when I started, and still can not find a fault with his statement.
2. While driving the "shop truck" or customer's vehicle do not "road rage", yell at other drivers, act like an idiot, honk or even flick a cigarette butt out the window. Remember, your employer's phone are typically on the side of the vehicle. Also, it is not much fun having someone follow you back to the shop because you shot them the middle finger. Did it once, will never do it again.
3. Shut up and listen when a more experienced technician is trying to
tell or show you something. When you are older, you will fondly remember
it and regret that you did not listen more.
4. Roll down a window or leave the keys in the door. In many states it is part of the fire code that requires the keys to be in the ignition or door if a vehicle is parked inside. By rolling down a window you can cut your chances of locking the keys in the vehicle to almost zero. Also, you do not have to roll down the driver's side window, roll down the passenger side or rear window. The driver's side window gets the most abuse and Murphy's Law states it will always break when someone else besides the driver tries to lower it.
5. Tools never get old, they only get worn out or neglected.