In a brilliant New York Times Op-Ed, Tim Kreider offers the following fill-in-the-blank form letter for artists:
Thanks very much for your compliments on my [writing/illustration/whatever thing you do]. I’m flattered by your invitation to [do whatever it is they want you to do for nothing]. But [thing you do] is work, takes time, it’s how I make my living, and in this economy I can’t afford to do it for free. I’m sorry to decline, but thanks again, sincerely, for your kind words about my work.