Make the parking visible and tell the story
When I have enough time, I tell this story of the nuclear plant and the bike shed. And then I explain to the meeting participants that I want that we stay focused on the important conversation. Therefore I have created a little parking space. It's there for all the other interesting ideas that are smart but not in the focus for today. I then take a big sticky note and write down: "Parking" on it. And then, I stick it on the wall.
I often conclude by saying:
You know, I'm a passionate guy, if you see that I go in too much details about something, you can ask me: "Daniele, I feel this is a bike shed topic, shall we put it in the parking?". And as you will do this with me, I'll allow myself to do the same with you.
If you don't have much time, leave the story but still explain that you have a parking spot for everything that needs to be remembered but is out of focus for today.
By the way, the story of the nuclear plant is one that is inspired by the term "bike shedding". If you want to look smart in your next dinner party read the Wikipedia article about it, it's a lovely conversation starter.