Bonnie ZoBell poses this question to a group of writers.
It's a question with many answers! Lorrie Moore recently said this about that-
"There is a vogue these days, Ms. Moore added, for story collections that are unified around a single theme. “My students are very attached to these thematic collections — they think that’s what the publishers want,” she explained. “But I never think of a collection as a form or a genre. I think of a collection as literally a collection — a temporal document. You put together what you’ve written over a decade, and there it is. I think each story should begin in a completely pure and independent way. Now, it will have things in common with other stories — it just will, because it’s coming from you. But stories have so much in common already, because they’re from one single writer, that there’s no reason to artificially make them talk to each other.”
Interview here-http://www.nytimes.com/2014/02/18/books/lorrie-moores-new-book-is-a-reminder-and-a-departure.html?_r=0
Give each story a number, put strips of paper bearing each number into a hat, pull them out one by one and lay them out in order of the draw in a descending line.
Unified theme concepts seem to defeat the richness of a short story collection, short stories being whole fictions if properly written. If you want a single theme, you might as well write a novel. Of course, that's only my opinion.