I find that I can only hold a conversation if I actually want to, that way my genuine intrigue concocts the questions for me and I just run with it.
Otherwise for the most part I can't help but give dead end responses and slowly die inside with discomfort...
But for general advice, if I had to continue conversation, I tend to say everything in an off the cuff yet nevertheless clear manner which opens up the possibility for them to either respond or ignore my comment, without me looking like an idiot in either situation as a result.
But then, you asked what to say, not how to say it... which is sort of invariably the first hurdle.
Like somebody else said, interests and hobbies and that for the win, but if you discover you share absolutely nothing in common, not only will that stagnate the conversation but I'm sure you'll also realise that there really isn't much point in talking to somebody you don't have any common ground with, even if its just a tendancy for the same satirical humour or something. (I could be wrong.)
Personally I find talking about the OC a greeeeeaaaat filler / starter come to think of it, as most people I know are obsessed, either by the greatness of its first series or the decline in quality of its latter ones. If you happen to be surrounded by a bunch of OC haters though, that would be very unfortunate and I would feel very guilty.
You could always reminisce. Everybody loves to reminisce. But then, that's basically saying that in order to hold a conversation, one must have actually led an interesting life or developed interesting opinions on which a conversation can be based.