You could try to drive the conversation towards topics that person is interested in, especially their hobbies, work or deep interests. Here are some phrases that might help:

What else is going on in your life? 
What else are you working on?
What else are you thinking about <with regard to so and so>? 
Anything else you wanna share? 

In addition, paraphrasing is a good way of showing that you're paying attention while helping them continue the conversation. This is essentially repeating what they said, but in your own words. Here are a few ways to paraphrase:  

“So what you’re saying is…”
“It sounds like you’re saying…”
“If I understand you correctly…” 
“What I’m hearing is…”
“Tell me if I’m wrong, but it sounds like…”
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