I am going to talk about my own and experienced contributors' experience with Alamy, the British mid/macrostock agency. Of course, if you registered to other agencies, these may apply. You can read plenty of interesting data or experience on the forums of the stock agencies.
What's the subject of your photo?
Your images must always have a subject! Ask yourself: what is this photo about? If you are able to give exact answer (it is about the oldest pear tree in my country etc.), then you are on your way. Many of us want to show too much in one photo, or simply shoot a wonderful view that was incredible to experience then and there, but looking at the photo later, you just cannot find the main subject, the thing that your image should be about.
I often read about the advice of he most experienced contributors that the background of a photo is even more important than the main subject. It means that be your subject fantastic, if the background is too busy, the overall impression looking at the photo will be disappointing.
The background should be as homogeneous as possible, especially in case of macro photography. Having some random objects in the background, your viewer will not know what to see. Distracting elements should also be retouched/removed from the photo.
Less is often more. Your photo must have a definite subject, and choose an angle where there are no lamp posts and such objects growing out from the model's head.
Only your best!
Ask yourself: Would I buy this image if I were the buyer? If your answer is no, or you are hesitating, then hit the delete button. The essence of stock photography is that you, as a very good photographer, are hired to do a job that you have already done in advance. In other words, the customer can choose from the images you have uploaded. Since the amount of competing photos is huge, the buyer will pay for the best.
If you upload a session, it is enough to include portrait and landscape format, a close-up and a wider angle one. Your chance to sell would be significantly lower if you upload the whole session with 10-20 photos of the very same subject. Why? Because the stock agencies rank the photos on their search pages by the number of sales of that image (among other factors). Say you have 10 images of the same subject, very similar composition. Every image of this session sells once, that is 10 sales. If you had uploaded only the best one, you would have also 10 sales, but that one image would go high on the search pages, while the 10 similars would sink deep.
Not to talk about how boring it is for a customer to look through pages of similar images while searching for the appropriate photo.
Be selective with yourself and upload only your best photos. The competition is big, so it is worth searching for your topic or niche, and shoot better than the photos on the first page. Or at least the same standard.
Tell a story!
Every stock photo is bought with a definite purpose by a designer, an editor or a blogger. The photo symbolizes something, tells a story for the people who take a look at the poster, the blog post, or the newspaper article.
Ask yourself: What is this image about? If you cannot give an answer, then you can safely delete that photo (except for the images with background patterns, which are used as design elements, not illustrations).
Not only awesome photos can be sold
Since stock photos are used on a variety of platforms, disgusting, ugly subjects are also demanded. And here I would like to mention the photos shot on ordinary places about ordinary things or situations. You do not have to always be creative to fulfil the needs. Several contributors tell their story when they only shot the neighbouring house or car or park, and made a sale with a significant price. It is the type of usage that defines what image will sell.
Actually, anything may sell. Alamy is the only platform, where only the quality of your images counts, the content can virtually be anything.
Watch out for your typos!
Read through the texts twice (captions, tags), and do not be lazy to correct yourself. It is true, that sometimes customers make mistakes entering their search terms, but it is less likely that a customer makes the same typo as you. Phrases can sometimes be written in different formats (i.e. close-up > closeup), so it seems a good idea to make a small research on the given stock site, searching for your terms. Study the first page as for titles (captions) and keywords (tags).
An advanced tool to analyze the keywords the customers usually use is Alamy Measures > All of Alamy (on your Dashboard). Just enter any keywords putting a % sign between or at the beginning/end of words (i.e. close%up), then setting the time frame to the max. 1 year, you can study the list of searches made by a selection of clients in that period.
Experienced contributors recommend including the commonly used wrongly spelled words, too.
Leave enough room for texts!
The negative space, or copy space, that we compose in the photo can easily be used by editors or designers to put some text, title on it. You do not have to compose all your images this way, but it is advised from time to time shoot with copy space in mind. A newspaper article will not need it, but having some negative space will definitely come in handy with posters or book covers.
Alamy customers pay the most for book cover usages (even gross $1000), so vertical images with some copy space can bring you good money.
You may find several interesting and useful pieces of information about stock photography by Kevin Landwer-Johan in this article.