The key to maintaining a conversation with allistic people is to realize they are expecting you to ask questions. So put on your "I'm interested" face, smile, and repeat something they said as a question.

For example:

Them: I went skiing last weekend.
You: Skiing? Cool! Who did you go with?

Etcetera etcetera.

But to be honest after a drink or two I usually get tired and return to my usual mode of talking about a related experience I had.