Flash fiction is the only form in which I write in which I often start with knowing how the story will end and then work backwards to the beginning. It is a great form for finishing with a punchline (and I am very fond of those).
Equally sometimes I will use Hemingway's classic six words story exercise to get me started. His famous example was For Sale: one pair baby shoes. A piece I used recently was He refused to kill the dragon. I've developed that into a flash story and it will be appearing as George Changes His Mind in my flash fiction collection, From Light to Dark and Back Again. The collection is currently in production with independent publisher, Chapeltown Books.
I find getting that "moment", whether it is at the start or the end of the story is crucial for flash. Flash, because of its limited word count, is also a great way to practice your editing and writing to a tight limit. I came into flash fiction almost by accident in that I had one or two short stories I couldn't do anything with. They were too short for standard competition entries, they couldn't be expanded (I hate padding and think it stands out like the proverbial sore thumb) so when flash took off as a genre, I realised these stories could fit there. I then came across Cafelit's 100 word short story challenge and thought I'd give that a go. I've not looked back since.
And flash is a great way to use incidents as one complete story, when they wouldn't be enough in themselves for anything longer. I also find it a wonderful vehicle for "mood stories" - i.e. I want to show one character in one mood, whether it is funny, dark (occasionally murderous!), sad or what have you. Give flash fiction a try - it can be a lot of fun.