Taking good stock photos is not as easy as you might think. In fact books have been written on nothing more than making appealing stock photography. Although I can't teach you everything about stock photography in this one article, I can give you some quick tips that can help you get started.
There are certain things that are accepted in professional photography circles that microstock agencies tend not to tolerate. Chief among these is excessive jpeg noise. Some other examples of seldom accepted practices included selective colouring, lens flair, sepia, and the use of other photoshop actions. If a photo with these techniques is exceptional then an agency may still take that photo, but your chances will be better if you avoid them.
Here are some basic concepts that tend to be successful in stock photography: photos of people, indoors and outdoors, groups of people always do well, business and technology concepts, medical concepts, silhouettes, and anything on a white background. This list can go on and on, most agencies have lists of their most desired content. Regardless of what an agency requests, good stock photography is only limited by your imagination and the capabilities of your equipment.