I am going to make a rather bold statement about short stories and say that the only thing that ensures an increase of overall writing income over a prolonged period of time is diversification.
The number one mistake people make is publishing a single short story in a single genre. The story never sells so they automatically assume there's no money to be had publishing shorter works. Not true.
Publishing short stories is a constant process of throwing spagetti on the wall and seeing what sticks. It's stupid to put all your hopes on a single strand. This type of mentality should be left to the novel writers as they slave for years over their supposed masterpiece. Short story writers don't have this kind of luxury.
You must experiment and you must make your work available to as many readers as possible. My sales increased once I stopped thinking that I was a sci-fi short story writer and started thinking that I was a short story writer. I branched out. I started new pen names and writing in all the genres that interested me. I also made sure that all of my work was available is every possible ebook retailer. Forget KDP Select. It will only hold you back.
I don't keep track of every sale. But I do monitor what IS selling and what's not. It's a constant balance of watching all channels and all the genres published in each channel and seeing what is picking up momentum. If I see, for example, that scifi sales are increasing on iBooks, this directs where I am going to be putting my energy when I start my next writing project.
Some genres just don't sell on some channels. Period. I think I've sold maybe a dozen copies of scifi stories on Barnes & Noble. Most of them to my mother. But on Sony and Kobo I have scifi stories regularly selling every month. Short stories may not be a gold mine, but getting $20 from B&N and $30 from Amazon and $15 from Apple... it does start to add up slowly but surely.