Jurassic London is one of a few publishers in the flash fiction market. There's plenty of advice about pagination and font selection already out there, so I thought I'd throw in a few tips about what we look for when we go through submissions.
Please note that these are my own opinions for the types of stories I like and commission. Other editors and publishers will look for different things, but these may be worth keeping in mind when you tackle a flash fiction brief.
Really short stories are still stories. A story should develop a character, have a clear setting or atmosphere, introduce a conflict (internal or external) and resolve it. I paraphrase China Miéville on this subject a lot - I want a story that "begins, middles and ends". The ending doesn't need to tidy up all the pieces (even a novel can't do that), but the reader shouldn't go away dissatisfied. One good example is Robert Sharp's "(0,0)" from Crossroads, which resolves the central conflict while still leaving an atmosphere of mystery. Jenni Hill's "Georgia", from the same chapbook, introduces an unusual character with an interesting problem. She develops the character, establishes a status quo, neatly overturns it and then ties the whole thing up - all while still meeting her tight word count.
That said, pick a thing. Introduce a conflict and resolve it. There's no room for sprawl with this kind of word count, and if you introduce multiple story lines, they'll either get cheated or left hanging.
Incorporating history - and our flash fiction is often alt-historical - also adds another level of complexity. Everything the reader needs to know should be in the story. You can't rely on them (or me) Googling or Wiki-ing or even recognising that they have a need to Google or Wiki whatever you're talking about.
There's no room for info-dumping. What's essential about the historical moment/person? How can that be introduced naturally, in a few words - through the setting or the dialogue? If the events are too complex, they might just not be suited for a story of this length. Marc Aplin does this well in "Son of..." from 1853, incorporating an intriguing historical event into the narrative.
To reinforce the above: the best stories don't feel short. If the story feels rushed, it is probably doing too much. Do something small well, rather than doing something big in a hurry.
On the plus side - the small space is an ideal opportunity to experiment with styles and topics that wouldn't be appropriate for a longer story. Tone of voice, experiments with language, narrative tricks, strange characters... flash fiction is great for having fun. Take, for example, Oz Vance's "Sketches by Zob" or Tom Loock's "A Tale of Cities Too", both in Fire. Neither would be appropriate for longer fiction, but use the short space to do something odd with language.
Please don't go meta. This is very much a house style thing, but worth mentioning. Nothing that breaks the fourth wall and, on that same note, I'm not a fan of stories about the writing process. It isn't my thing, it is very, very difficult to do well and it is far more common than you might think.
One more note on house style, that's specific to the "Pandemonium" world. This setting requires stories which only have a touch of the fantastic, and sometimes not even that. Part of this is because the Pandemonium books all share a world: if one author unleashes a plague of zombies, that plague of zombies can seriously constrain everyone else. Similarly, we're not looking for epic fantasy - nothing that sets up a world where the presence of magic or the supernatural is taken for granted. Laura Graham's "Letters from Auld Reekie" (1853) is an example of the low-key weirdness, and the polite world-sharing, that we're looking for - the supernatural elements in her story may or may not exist at all.
(If it helps, all the chapbooks I've used as examples are free through the Kobo links provided.)
None of this is meant to be discouraging. There's no part of the editorial process that I enjoy more than reading all the stories that come in: they are weird, wonderful, varied and invariably interesting. Keep up the great work.