Recently I finally managed to upload a model of a retail plaza to the Google 3D Warehouse. It took months of method, speculation, and temper + computer problems to deal with, but eventually it worked.
As with that final result, I've decided to write some tips that have evolved out of the many mistakes that I've learnt from as well as my experience.
For even further information, check out Google Sketchup For Dummies. I'm not kidding, it's a real book and it has served me well on various shortcuts and photo-texturing methods.
So, here are a few tips that I can bring to the basic discussion here:
1. A major factor in modelling for Google Earth is file size. A normal model cannot exceed 10mb in size. It's simply rejected if one tries to upload it.
To reduce the file size, make your model as simple as possible. The standard building outline is easily enough, and no interior rooms or decoration is required. I'd recommend not exagerating outdoor features such as window sills, window panes, frames, or steps. The lines and parts that come together to make a face or object are called polygons, and useless ones can be deleted.
Making a simple building is fundemental, but not the most space-sucking. What really increases the file size is this: Photo-textures.
This comes from my own experience. When taking your photos to texture onto the model, use the smallest-possible format they can be saved in on your memory card. For instance, if you have a 12 megapixel camera, go to the options and select the photo size to be 1.3 megapixels instead. The photo won't be huge in result. It's alright if you can't zoom in to it on the computer that way, because it's intended to be a texture, not a detailed piece of art intended to be blown up to a large poster size. Further more, you should then crop your photos to the exact wall or part of the building you intend on texturing, so you're only using a small percentage of an already small photo. That, or you can compress it down on the computer. I should also note that cropping it reduces the amount of time you have to arrange and position the photo texture on to the model.
Doing this will slow down the progress of the size of the whole thing altogether. And always make very well sure that it is as it is. For instance, if the computer says it's a little too large, use a trial and error process where you try to upload it anyway. If the 3D Warehouse agrees with your computer then it's safe to believe it's too large. I know this myself - I didn't do that once after finishing that exact model last summer and believed the computer even though I could've made sure, and in my wrath I deleted the whole thing.
Using the smallest photos possible and keeping it simple should keep the model under 10mb. (That's 10 megabytes).
2. Be inconspicuous. What I mean is when you are on location attempting to get your photos to texture. Some people feel odd when there's some person standing near a normal building, taking photos of walls...or something else more suspicious. For instance, I couldn't continue to take photos of a nearby McDonald's because the manager saw it as a form of trespassing or unauthorized visual depictions of the exterior of the building. It does sound ridiculous, but when I explained the Google modelling thing to him, he seemed to believe I was with Google myself and told me Google didn't have McDonald's permission. Google doesn't really need permission. Not for that type of building anyway. And there are millions of photos taken by millions of people who photograph McDonalds for whatever reason. I would agree that taking photos of a private residence wouldn't be acceptable and it's the owner's decision to model his property, but a commercial building can be photographed and modelled by anyone. I don't believe there's a law against it (not here anyway) or else I think Google would have made sure to make that clear to model-makers. Tonnes of people model commercial buildings for Google Earth. Look at New York City. I can remember finding a model of a shopping centre in Michigan. So to avoid confrontations from suspicious people, my advice is to time your expeditions at the right moment. Photograph the building on Labour day or on a holiday where nothing's open. You get the advantage that there's no cars or people in the way. Or do it whenever they're closed.
The thing is, people will get uneasy when there's a photographer who has no noticeable subject to photograph. Especially if they realize that it's a building. For all they know, they could be taking photos of people through the windows.
3. Be thorough. Texture everything. It doesn't all have to be photo-texture. You can 'color' the roof grey or brown or whatever. Any place that you couldn't reach with your camera you can simply color with a color that's close to the overal tone of the photo-texture.
One last good tip is the auto-measure thing. What I mean is you can draw a particular line, type in the measurement, hit 'Enter' and it auto-completes the line to the exact measurement you keyed in. This can also be applied to pushing/pulling, circumferences, angles, and more. It all depends on the unit of measurement you use.
So those are my personal tips for building realistic, Google Earth-ready models. The big, important one I think is the file size. Also be considerate in what you do model. Landmarks, public and commercial buildings are the best candidates for this. Keep away from private or independently-owned properties. They can model their buildings themselves if they want.
For all the modellers out there, I hope they get a lot out of this and I encourage them to keep doing what I think makes Google Earth so realistic and cool to use.