When to use it
Use it when you already know the answer is no, whether it's a committee, a side project or an event. A warm thank-you plus a clear reason ends it kindly. “Let me think about it” only delays the no and leaves the other person waiting.
Example
Colleague: “Want to join the events committee?”
You: “Thanks for thinking of me. I'm going to pass so I can focus on the launch.”
Colleague: “Totally understand.”
In an email or Slack message
By email: “Thanks for thinking of me for this. I'm going to pass so I can stay focused on the launch, but I'd be glad to hear how it goes.” Short and final.