If there is such a thing as etiquette of using your car’s horn, what would some of its finer points be? ~

Honking at the idiot veering into my lane while texting.

Use it to alert other drivers, not as a commentary on their driving expertise.

1) To prevent an accident
2) A light tap as a greeting, if it doesn't compromise safety 

A short beep for a friendly warning of danger.
A longer beep to alert someone who is clearly distracted and not seeing what's going on.
A huge, long bellow for someone who's going to cause a loss of life if they don't act instantly.

Flashing lights to oncoming traffic if one has passed recent danger, such as a big pile-up or an animal loose on the road.

Short beep for someone who is not paying attention when the traffic light turns green.

Slightly longer beep for someone crossing the street against the light but who is distracted and doesn't realize you're almost on top of them.

Long beep when someone is entering the highway without waiting for a break in traffic and thinking they have the right of way and you should stop an let them in or when someone is changing lanes and is too close to you to void a collision.

use it only when you need to