Open writing competitions

Today is the first day that was just for me in ages and I was relaxing with a cuppa I flicked through the latest edition of Writing magazine this morning and came across the 2008 writing competition calendar. I noticed that almost all of the story competitions have an open theme. You can write about whatever you want. How on earth do you come up with a winning entry - or any entry come to that? I think I now know why I've never entered a competition that didn't have a theme before.