Wiring I won't bother with an actual schematic here because this is another very simple (and cheap) modification.  The idea is to isolate the rest of the guitar's electronics from the string ground.  Between these two sections you wire in a small value cap and a relatively high value resistor in parallel between the string ground (i.e., connection to the bridge) and the rest of the circuit.  A typical set of values might be a 0.02 µF capacitor with about a 250k Ohm resistor.

If you happened to be holding a typical guitar in your hands at the time when ground in your amp happens to fail, you would become the path of least resistance to the ground.  With the modification described above in place, the resistor blocks the full current from reaching you (well, maybe; personally, I have some doubts) although the capacitor would allow you to feel the voltage change.  If you feel something tingling (or worse), you would probably think to get away from your gear before it gets you.

So how could your ground go bad?  Well, I'm sure there are other ways, but water will do the trick.  That will short the circuit under the right (or wrong!) circumstances.  I witnessed this first-hand when I went to watch a friend of mine play several years ago. He and his band were playing under a bandstand in a park after it had rained.  Even though things were sheltered, the ground was still very wet and it was very humid as well.  Throughout the night, enough errant charge was able to arc through the circuitry to repeatedly shock my friend.  At first we thought he was just putting us on and spazzing out for effect, but apparently this really hurt... though not enough to stop him from playing for more than a couple beats.  Thankfully, I've never experienced this.