This is just a short post that begins with two anecdotes from this semester:
1. Person asks me to review a book that I'm actually interesting in reading. They say "we need it in six weeks." I say, "Sounds interesting but I have a number of projects that I've already committed to over the next several months. I could do it, but only if you can wait until August." They respond, "Perfect! Where shall we send it?"
2. Person contacts me asking me to blurb a book. I respond about commitments, say "Not available till July." Response: July will be great; do you want it in hard copy or pdf?
Something that both of these have in common: they both respond to my offer to get back to them about four times longer than they want... within 15 minutes.
Lesson from midcareer: When you are doing work for free, you have a lot of power to set the conditions. You can say no, but if you want to say yes, you can say yes on your own terms.