I was talking to one of them a few months ago. She's a clinical social worker in private practice, and she told me that when she's contacted by a prospective client, she always sets the first appointment at least two weeks away from the date of first contact, even if she has availability before then.
"Why?" I asked.
She smiled. "Because I want to let them know that I'm a therapist who is worth waiting for."
After we spoke, I played over her idea in my mind. Would a similar tactic work for writers? I didn't see why not. In fact, I saw many advantages to not dropping everything to chase after a new client.
First, having a waiting list projects an air of confidence rather than desperation. If you spring at any hint of new work and bend over backwards to get the gig done sooner than immediately, your behavior communicates that you are willing to do just about anything for paying work. This gives him or her the upper hand in negotiations and all but guarantees a series of low-paying quick turnaround assignments. I don't think that's what any of us are looking for.
Second, having a waiting list allows you to maintain control of your schedule and your work/life balance. If taking on one more project this week will push you to the point of overwhelm, simply let the client know that you'd love to work with him/her but won't have any time on your schedule until next week. Most clients are impressed by the fact that you're in such high demand that they won't mind the wait.