A few tips my dad and I have learned over the years:
* - As stated, green lanyards can only trade with children, though I don't know if there is an exact age cutoff or anything. Now, I've never been carded by a green lanyard-bearer, but I've never tried either.
* - Resort-hopping (especially the Monorail resorts and the Crescent Lake/EPCOT resorts) is a fun "downtime" activity anyway and asking in each gift shop/concierge station to see their pin board/book makes it extra fun. One of my fondest recent Disney memories is going over pins while enjoying Lapu-Lapus at the Polynesian.
* - There is a pin board/book/something else entirely lurking somewhere in each park. You just have to ask around. Our favorite, the most absurd, is the pin stroller they wheeled out for us in Animal Kingdom.
* - The two most popular pin trading hubs are the kiosk in front of the dancing fountain in EPCOT and the big pin trading store near the far end of Downtown Disney. A warning: this is where the Pin Trading Is Serious Business people tend to lurk. They will usually have huge books full of rare pins and may your deity of choice help you if you ask to trade one of your cheap pins for something that happened to catch your eye. At best, they will laugh at you. HOWEVER, a few of them are sympathetic to casual traders, and may have spares of rare pins. IMO, the risk is worth it.
* - As with so many things in WDW, it pays to show up early for things. Like the soaring condor and mischievous raccoon, pin traders are opportunistic animals, and all these lovely Cast lanyards, boards, books, strollers I have mentioned are usually picked over by mid-afternoon. If you want the best chance at scoring something rare, get up early.
Granted, my advice is a little tongue-in-cheek, but I hope this helps and happy trading!