I'm still new here, and being a full time student unfortunately does not leave me that much time to shoot stock photography. One thing that has helped me is figuring out what buyers want. There are many good ways to do this, mainly by looking at the top sellers for the month, but what helps me is to try and think like a designer.
Now, the tricky part is that all designers don't think or work alike so that makes it difficult, some like premade backgrounds that they can add text to, others like to take objects from multiple images and combine them together, the possibilities are endless. One person I knew that used stock imagery pretty much always would buy already complete designs, then add text elements into them.
So do I know what sells? No, not really, but like everything else you can do in life, you just have to learn.