One thing I have found quite useful and a real timesaver is to keep a library of shadows. I keep a few basic shadows on file and repurpose them instead of having to recreate them from scratch each time a new image comes in. This is particularly useful for car images, but you may find other uses for them. I usually keep them as a fairly large file, but the nice thing about a shadow is that it can be resized quite considerably, 200% or more, and the image doesn't really degrade all that much. Add a slight Gaussian blur and a little noise to it, and you're back in business.
One big tip for creating shadows is: don't restrict the shadow to the shape of the object. By this, I mean create the shadow as if the object you are making the shadow for isn't even there. Do not create the shadow with an inverse selection of the object
I say this because if you ever erase or shift the position of the object or shadow, there will be a hole or line where the shift has taken place. You can see the problem in Figure 2-53.
Create your shadow so it is behind your object, and extends itself throughout the image and isn't restricted by any portion of the object, as in Figure 2-54. Also, if you build a library of files and you reuse your shadows, having full shadows as opposed to shadows with previous shapes cropped out will make them more adaptable; a shadow cropped for a specific object will likely be useless.