Honking Etiquette

We've learned over the past couple days what is and isn't appropriate 'honking behaviour'.
As far as we're concerned, the guide to successfully honking at a cyclist can be broken into three themes:

1. Purpose
2. Length and Type
3. Direction of Travel and Location Relative to Cyclist

1. Purpose of the Honk
Are you saying "Be aware, I am approaching you from behind"? If so, this is generally appreciated if done correctly (see location relative to cyclist).
Are you saying "I see you there cycling, with all your kit, making your way cross-country. Good Job!". This too is appreciated (see direction of travel and location relative to cyclist).
Are you saying "I hate you and I want you to die"? This is less appreciated, and if so, please ignore any of the following information.

2. Length and Type of Honk
The best way of attracting attention is a couple quick beeps of the horn. <beep-beep> Anything other then that can draw too much attention, and cause alarm. <BEEP> or <beep-beep-beeeeeeep>

3. Direction of Travel and Location Relative to Cyclist
At no time, should someone honk when directly beside or immediately behind the cyclist, even if it's in frustration! It's startling, and at the stage of a vehicle passing, any kind of disturbance can cause the cyclist to become unbalanced. An unbalanced cyclist can lead to problems, especially in high traffic.
For vehicles traveling the same direction as the cyclist, honking 20' after passing is preferred.
However, if you're driving a big truck and want to give a heads up, honking more than 40' before passing is great too.
For vehicles traveling in the opposite direction as the cyclist, honking 20' before passing is preferred, but waving or hooting is even better.

Two final thoughts:

Firstly, if you do become frustrated, save it... and if you cannot, let out your <BEEPity-BEEPs> well past the cyclist. Flipping the bird is also fine.

Secondly, a spot where it's especially nice to show support to a cyclist is when they're stopped on the side of the road to take photos of the summit sign they just owned! Here, honks or hoots of any sort are fine. Throwing of rocks and boots is still discouraged.