Not only wouldn't I sign an NDA, I've also started getting very careful about when I might contribute my own wisdom and experience. Far too often, I've found those sorts of meetings to be as much about trolling for free advice as anything.
I explain to these people that consulting is one of my services, and the price is on my rate card.
Personally, I've always been happy to give modest amounts of free advice. My standard rule when I was consulting was that I was happy to meet with anybody for lunch. The overhead of signing a contract isn't worth it for an hour or two, and it was fine free advertising. It's no bad thing to have a lot of people know you as that smart person who really helped them out in a pinch.
I explain to these people that consulting is one of my services, and the price is on my rate card.