I think that if you want to grow as a writer it's really important to write everyday, even and especially when you're not feeling creatively inspired. So, on that point if you want to get good at writing short stories and getting them published or winning at competitions, you need to write in a broad range and explore with different styles and themes.
A really good way to do this is to find short story competitions and magazine submissions that are themed. At first it's really hard having your work dictated by a theme that's way out of your comfort zone, but it's a challenge worth persevering with as it will broaden your range.
If you'd like to do this, and your passion is fantasy fiction why not try out Fantasy Faction's monthly themed writing competition, which I've mentioned before as I decided to give it a go. This is a great website that also has articles on useful writing tips, book reviews, interviews with top authors and the latest fantasy fiction news.
Each month there is a new theme, the great thing about it is that you can read all the entries and the voting is public too which allows you to see how well you have done. This is a double edged sword as my first entry failed to secure any votes which was disheartening. However my second attempt came 5th out of 13 entries which I was very pleased with.
Writing to a theme also gave me to confidence to try out for Siren's Call ezine which holds regular themed submissions. Miraculously I was successful and was published in their February edition, Whispers in the Dark, which you can read for free here.
It just shows that if your willing to flex those muscles who knows what you might achieve.