I was in Rome for two weeks shooting a documentary and in my down time I was stumbling around on Philip Bloom's website when I came across this tutorial:
philipbloom.net/2012/02/01/the-art-of-lens-whacking-real-lights-leaks-and-ones-done-in-post/
The technique sounds difficult, but it's really very simple and the results speak for themselves. I edited together the film below from footage taken throughout one day and night.