Designing an icon for use in a Google Earth project requires thought about how that icon is to be used. If it is to be used once, as in the Eagle icon in 'Explore Artic Drilling' then you can design something with unusual shapes and colours to be eye catching, I liked this little fellah found in a good article on icon design for desktops here.
However, if you are using a number of icons to mark multiple of points then your design must be different. You'll need to make the icon easy to differentiate from other icons and still visible when small - reducing size reduces clutter. You will also want to put common features into icons that should be grouped together. Lets look at an imaginary example
Taking the excellent explore artic drilling project I mentioned before I took the single use icons they used to mark 'read this for more information on wolves/eagles' and copied them around to simulate a project where you are trying to show sightings of these animals in a field study.
Immediately you can see that the blue borders and use of the same square shape limits the ability to differentiate between the placemarks. I also think the three colour design (brown, white, blue for eagles and grey, white, blue for wolves) looks 'busy' when the icons are repeated a number of times.
However, the blue borders and white backgrounds do strongly mark out the 2 types of placemark as having similarities, which is good as they both represent animal sightings.
In my version I have drawn some new icons (notice I'm no graphic designer!), the two colour design reduces busy-ness and the simplicity of the icon allows it to be reduced in size which allows more positional information to 'read' by the user. Also, the octagonal shape of the wolves (based my late grandfather's favourite soccer team logo by the way) compared to the rectangles of the Eagles allows more visual information for the user to help differentiate the placemarks.
In terms of linking the animal groups together, the light yellow eyes of both animals and the similar background colour sends the message 'these icons are in a group'.
A lot of these rules of thumb come from a major influence of mine Edward Tufte, These seem to be notes from one of his lectures on how maps should be constructed. One of his key points about any informational graphic is that the comparison of the data is key to good design, thus in the Explore Artic Drilling project it is the wealth of oil icons compared to pristine wilderness nearby that is striking.
So you now have some ideas of what makes a good, multiple use, icon but you have no idea how actually to do it. Rest easy gentle reader, I will cover that in a later post. If you want to see how the project works at other view points the .kmz link is here.
I hope that no one reads criticism of the Explore Artic Drilling project from this post, none was intended, I just used it as a handy source of a problem and icons to illustrate an issue.