For the first challenge I had an idea and wrote it up quite quickly, I spent more time on pruning it to 75 words than actually writing it all down, but then once it was posted (and I was very happy with it at that point) I didn't like it and wished I'd spent more time on it.
So now, when I have an idea (fairly soon after the competition starts) and then write it, prune it, but then let it fester for a while and see if I still like it. Usually I just rewrite a few words, swap a few lines around. If I'm feeling brave I remove a whole sentence and add something else.
I've found that the idea usually comes quite quickly, but then it takes a little while to solidify into an actual story. One of the things I don't seem to be doing is making a story, I'm just writing a paragraph about something and tacking on a rubbish final line to try and explain it all.
I did write one this month with a start middle and end, but I chose the other story.
How I come up with the idea is a mystery to me, this month I tried to think of things that transform, I toyed with the idea of geographical shapes for a while, but then prefered the idea of an idea (or I thought of a thought).
Wasn't sure if it was ready for posting, but will be a bit busy this month* so thought it best to get it posted.
* This is my last month of paid work as redundancy has hit, so I'm going to have a couple (at least) of months off to see if I can write full time. If I can complete 75 words a day it'll only take about three years to write a full length novel. No probs!