It all depends on whether you are talking about trading with other guests or cast members. You will see a lot of cast members wearing lanyards. You can approach them - ask to see their lanyard. They will trade with you as long as the pin you are trading is not currently on their lanyard (however they will usually give small kids a break and still take the duplicate pin).
Trading with guests is a bit trickier. Feel free to stop by the Contemporary on Friday nights or the Polynesian on Saturday nights to see trading in action. These meets start at 7:00pm. There are also pin meets in the Magic Kingdom on Tuesdays (I believe at 1:00pm when Scoop Sanderson does a whole program on pin trading). Also a meet at Pin Central in Epcot on Fridays. I think that one starts at 1:00pm too.
Be sure to check out the Collectors Forum for up to date pin information. Also feel free to email with questions.
Cyn