For me, short stories are hard to get started. My most successful stories, including "Switched," which appears in the anthology The Magical Muse, felt like they wrote themselves. I had a powerful feeling for the world and the characters I was writing, and a clear plot in mind. This allowed me to condense the elements of the story and get right to its heart without sacrificing the clarity of the plot or the strength of the characters.
Still, it's tricky with fantasy stories because often the worlds we create in fantasy or scifi are unique to our imaginations. This is why many fantasy writers spend so much time "world-building," or writing about the setting of their stories or novels. For short stories, our world-building needs to be minimized as much as possible. We need to suggest the nature of our world in as few words as possible, or else our "short story" quickly swells into a novel with a limited, short-story plot. I handle this by trying to keep the world entirely within one character's perspective. If that character knows little about the nature of magic or magical races, than don't give the reader an explanation either. It's often more dramatic and emotional if characters don't completely understand what's happening to them, or how things work. Life is often bewildering.
Short stories are also good way for writers to explore different genres or writing styles without committing to an intense project. If you normally write serious, dramatic novels, you could try writing something light-hearted or funny in a short story. I've written short stories about minor characters from my WIP. That allows me to flesh out these side characters and give them more depth.
While writing short stories is a challenge, it's also a fun way to work through ideas and find inspiration. After I finished my last novel, I was a bit lost and didn't know what to write next. I had several ideas bouncing around in my head, but each time I sat down to write something, nothing seemed to come together. Finally, I wrote an emotionally intense short story set in an elaborate fantasy setting. I became so fascinated by the world and the characters, that I've started writing a novel set there!