I have been researching electrostatic discharge when it comes to safely handling PCBs, and I'm having trouble understanding how I should ground myself. Many people say to just touch any conductive structure in your vicinity, but I don't understand how that would help. The way I understand it (and this could be wrong), as long as the PCB is at a different electrical potential to my body, current can flow. When the PCB arrives in the mail, it could be at a lower, higher, or the same potential as earth ground. If I ground myself to earth, there is no guarantee that the PCB will be at the same potential.

I have also seen people say to ground yourself to the thing you are working on - this usually comes up when people are talking about PC repair or construction. Even in this case, wouldn't grabbing a new PCB out of the bag run into the same problem (say if you were replacing a PCB in your computer)?