When you don't know what to say next, act like a parrot and repeat the last few words of the person's sentence, either plainly or in a slightly questioning tone. If the person is interested in talking to you, he/she will be compelled to continue talking, repeating the sentence and usually providing more details, etc. Remember when you used to have those endless "why?" conversations as a kid? This is the grownup version, except it's not obvious what you're doing since you don't keep asking "why?". Try it on your friends and coworkers. Then try it while gaming. It's pretty funny how long you can keep a conversation going without contributing a thing. This is from Leil Lowndes and it works.