Noone knows exactly how people get a PIN code email or snail-mail postcard, but there are a few things you can do to increase your chances:
-Make sure you've signed up for a login on the WDW website. Even if you have one, it wouldn't hurt ot go in and update your info. Make sure to check the box where you say that yes, Disney and its affiliates can contact you with promo material.
-Request a new Vacation Planning DVD. Make sure to answer the question about when you're thinking about going on your next trip.
-*Maybe* even go so far as to book a trip and then later cancel it... (Though I admit this one is pretty drastic) A friend of mine got a PIN code postcard in the mail last year, and the only reason Disney had his name and address at all was because on the trip my friends and I ended up having to cancel, the second hotel room was in his name.