When you write articles, always be thinking about putting it all together into information products that you can sell. Even if you are working in more than one niche, focus on what you are writing so that you can create products easily. One way is to write about 10 articles in one subject area. Combine several of these articles together into a short report that can be used as a lead generator. Then expand the short report into an ebook that can be sold. Make a recording of what you have written and include that with your ebook to give it a higher perceived value. You can also put together a workbook that people can fill out as they read your ebook or listen to your recording.
There are many ways to profit from your online writing. Focus your time and attention on the area you want to specialize in and you will be able to create many products and produce a steady stream of passive income.