Advice on writing? Not my forte, place of expertise, or indeed something I have a full understanding of. Yet if pushed I could probably summarize my approach to writing flash fiction as in seven steps.
Thankfully Michael Arnzen has written a far more engaging and reflective piece featured by the folks at Fiction Vale. Entitled Making More Out of Less: Strategies for Writing Flash Fiction. One point that I found particularly insightful was the section Exploring the Gaps
“Employing the trick ending is more common in some genres (horror) than others (romance), but most flash fiction stories seem to end with a punch in the gut that leaves you realizing you had taken something for granted all along. Short-shorts take shortcuts all the time, building their stories on reader assumptions, or leaving so many gaps that the reader must read between the lines to make sense of the story. Exploit those assumptions to create surprises.”
Hope it proves of interest/help.