Last Monday there was an article by David Gaffney in The Guardian about how to write flash fiction. There was a lot of good insight in it, and I encourage writers to check it out:
http://www.guardian.co.uk/books/2012/may/14/how-to-write-flash-fiction
One of my favourites was his note about making the title work for a living. When you only have 1000 words, the title can be a godsend- it’s like getting two or five or seven words scot-free! I also liked his bit about making the last line ring like a bell. Flash fiction is often compared to telling a joke, in that you’ve got to have just the right amount of set-up and a killer punchline. A solid last line can turn a vignette into an actual story.