I knew enough about sci-fi to not try the hard variety right off the bat. I had written some mainstream military/war, so I figured military sci-fi wouldn't be too hard (haha). Several hundred words later, Zento the Mercenary resulted.
The basics of writing speculative fiction apply to each genre. You need a strong central character, a really cool setting, and a thought provoking plot. The character, Zento, was a classic space mercenary with a couple of exceptions. He wielded a crossbow and did it for the thrill, not the money. The setting was far from flashy, not considering the literal sparks, one of my larger flaws. Zento the Mercenary is definitely flawed, although not too horrible compared to my other pieces at the time. The plot was simple, as most flash fiction is. Most sci-fi, however, has a fairly complex plot. Military sci-fi is somewhat lacking in the area, but still needs to be well-constructed. Luckily, I learned from my mistakes.
Romance is another genre I have little experience in. While writing the few, all unpublished romance flash pieces I had to delve into my real emotions to make an impact. Perhaps one day I'll post a short romance as a Sunday flash.
It can be hard writing genres you aren't adept at. Unfortunately, if you want to be a great writer you need to. You don't want to be the person who can only write one genre. Mastering them all is far from necessary, yet delving into a dozen of them can put an ace up your sleeve.
I leave you with a short Hunger Games-inspired poem I wrote a while back that has a link to the chance-taking of writing new genres.
Roll the dice
Take a slice
Of the pie of life you savor
"May the odds be ever in your favor"