First things first: It's like 13E11E said: It comes naturally.

However, it's always good to have a cheat sheet. I came across a very fitting analogy. Imagine a game pad. You can zoom in or out or switch left or right to a related topic.
An example:

You're talking about your last vacation. He/she tells what they did, what exciting things he/she experienced.

If you transfer the analogy, you can imagine the possibilities:

You can explore the exciting event in more detail, you can ask what else (in general) he finds exciting. Talk about something exciting that comes to your mind; the main thing is that it is appropriate.

The Best way however is reflective communication. Try building an emotional connection.
"Oh! Sounds like you had fun man!"

Lastly: Don't try to keep the dead alive. When the conversation is over, try not to keep it alive artificially.