I like clever sayings and wise proverbs, the shorter the better, so I wanted to work a number of them into the book. There are 45 of them in there, and for the most part I think they are at least relevant if not insightful. However, I admit that a few were just so good I bent the rules to include them. I hope the add a little spice to the book, provide useful perspective, perhaps educate or even provide a chuckle to readers.
When I felt stuck in the writing, it proved to be a nice technique to would spend a little time poking around the web for good quotes to go with the topic at hand, or sometimes to fill a gap elsewhere (from a list of places I wanted to find quotes). Usually, a few minutes searching was boring enough that I could return to writing afresh thanks to that slight diversion.
Also along the way I kept a file of good quotes that I had no spot for, and sometimes would find gold there later in a completely different context. I was very unsure if these would make it past the editors, but throughout the entire book there never was a single remark about any of the quotations.
My personal preference is for short quotations, and I wrote a simple script to sort a web page by word count (of paragraphs) so I could skim the shorter ones quickly. A friend with a law degree advised me to strictly avoid song lyrics as those are particularly well defended. The other filter I applied was to avoid quoting people whom history has not been kind to, because we can move forward without their contributions.