Stock is a business based on creativity. Knowing what subjects to shoot and how to diversify your collection is paramount to creating a good income from stock photography.
At the recent PhotoPlus in New York City, I was privileged to be on the "Do It Yourself Creative Research" panel. My approach was to discuss how to do creative research when the photographer may not have the benefit of working with a larger agency that provides trend data and shoot lists. I also look at keywords as the "real estate" of the stock industry.
Here's the presentation.