In my AGU talk the other day I end by saying that the most important point to remember about design in virtual globes is that although I have lots of ideas about what works in Google Earth to make good design, testing is the final word on effectiveness. I recommend Steve Krug's book 'don't make me think', it was an inspiration to me about web page design at the time where Google Earth was a twinkle in Michael Jones' eye and most of the principles map across to Google Earth projects.
I find the technique of testing I outline has been written up in peer reviewed literature and is actually known as hallway testing from the idea that if you pull 6 people randomly from any hall way and quickly use them to test your application/web page/Google Earth project you can identify 95% of usability errors.
I also looked up paper prototyping, a technique that I came across in the Open University (UK) where you prepare web pages scribbled on bits of paper and model how it would work as a web page by switching paper sheets when the paper buttons are pressed. Not only effective but a lot of fun. Robin Good has a good intro.
Like tidying your office and managing your time properly testing is not something people are naturally drawn to do but they reap the rewards if they do.