Shoot photos that sell
That's by far the most important point. Would you pay for an image of your neighbors mother-in-law? Or of his dog? Of course not! No one would, perhaps not even your neighbor himself.
Likewise professional photo buyers don't care for that kind of images. What they are looking for are photos that illustrate concepts, like career, relationship or retirement. Business related photos generally sell very well. Photos of handshakes sell well because shaking hands is a universal, widely understood idea that can be used to illustrate negotiations, contracts, treaties and even things like breaking-up or divorce.
Travel photography can sell well if it can be used to illustrate concepts. For example, a photo of the Houses of Parliament in London can be used to illustrate democracy or governmental topics.
Avoid legal pitfalls
Most stock photography agencies have strict rules regarding images of people (if the people in the photo are recognizable), property (if the image of the property can lead to its owner, e.g. a license plate on a car), and trademarked logos or items anywhere in the image. If in doubt, don't submit such images. If you want to sell images with recognizable people in them, all agencies will require you to provide so called "model releases". A model release is a document with which the photographed person permits you to sell the image without need of compensation. Obtaining a signed model release from ordinary people is next to impossible, so you might be better off to either weed those images out or hire professional models.
Keywording is the key to success, literally
No matter how good your photos are, they won't sell if no one can find them. All stock sites let you tag or keyword your images. A good approach to keywording is to answer six simple questions for each image: Who? What? Where? When? Why? How?
For example, let's suppose you have a bunch of nice healthcare related images, shot in a hospital. Answering "Who?" you might find "doctor", "nurse", or "patient". Answer "What" to come up with "lancet" or "stethoscope". Answering "Where" yields "hospital", "waiting room" or "theatre", while "When" gives "morning", "afternoon" or any other time of day or year. Ask yourself "Why" to evoke concepts like "sickness", "comfort" or "patience". Finally "How" can refer to the photographic technique involved: It could be "black and white" or "monochrome", it might be "blurred" et cetera.
Keep the noise down
Always keep in mind that the end user of your image may want to print it out eventually. The larger the print size the more noticeable noise will be. Noise is induced by your digital camera's sensor and is something digital photographers have to live with, much like traditional photographers had to live with film grain. Generally speaking the smaller (area wise) the sensor size and the higher the ISO sensitivity the higher the noise will be.
