I’ve driven a few technical editors to drink with my challenging writing: tenses and I don’t always see eye to eye, a few split infinitives add rhythm to an essay, and gender-correct “he or she” or “s/he” is correctly written as the singular “they” in my none so humble opinion. So I was enlightened to discover the secret of prepositions in this Dr. Language column: http://www.yourdictionary.com/library/drlang001.html