While they could prove his revelations are not true (assuming that they aren't), this would have a serious disadvantage, though. It would inform anyone at odds with them of capabilities they don't have. Note, I'm not saying NSA doesn't do things Snowden said they do. In fact, they're not the only ones that do this (FBI vs. Martin Luther King case was referenced a few times), but if it turned out they don't, it'd be quite a compromitation, because they're, you know, an intelligence agency. They're kind of supposed to do exactly this sort of shady work, and it'd be really embarrassing if it turned out supermarkets and data mining companies do it better. Also, the fact that NSA is lying (they almost certainly are) doesn't mean Snowden is telling the truth. Quite the contrary, I have a suspicion both are lying, for different reasons and in different ways. This whole situation does not smell fishy by that point, it reeks like an old trawler.
So what should we do? What we always did. Let's face it, we're being watched, no matter where or who we are. NSA is just another pair of eyes sifting through our data. Along with data mining companies, ad sites, Google, every major shopping network, probably half the websites we visit and more than one foreign intelligence agency. Just be careful what you say on the internet, and don't say things you're not ready to say in public. Sensitive data doesn't belong on internet anyway. I don't think that an average person is at risk of having an active defamation campaign started by NSA (or any other intel agency) for one simple reason: money. NSA (and not only they, IIRC, Mossad pulled this out with some Iranian scientists, too) surely can destroy any chosen person's credibility, but it costs money, probably a whole lot. Therefore, I don't think they'd do such a thing without a very good reason. Travel restrictions for calls to Middle East are cheap; it's just a phone call/some paperwork sent to TSA. An active campaign against a person much less so, probably involving quite a few trained (and well paid) specialists.