On recent trips, I've noted fewer and fewer cast members wearing lanyards. When I asked a few folks, they mentioned that there was a feeling of being abused by some pin traders that had lead to fewer CM's doing it.
That being said, a polite, "May I look at your pins?" has never let me down.
Depending on your feelings about eBay, you can find some good deals on pins for less than $2 each to trade at the park. I've had good luck with lots of eBay sellers over the years, especially when it came to pins.
One new (at least to me) guest behavior that I find apalling is asking another guest to purchase a new pin to trade for one from a book or lanyard. Why should I buy you a brand new pin for one that you probably traded for off a CM lanyard or got from another guest?
I told one guy what I thought of him for doing that and it was not pretty. Basically let him know he was lower than pond scum.
My best pin trade was with a six year old girl. Nothing special, but it was a simple guest to guest trade and it made her smile when it was all over.
And that's the real Disney magic.