I have friends who give their kids a set amount they can spend, and tell them they can keep anything they don't spend. Mom and Dad determine the amount and keep track of purchases. It doesn't save the parents any money, but it sure cuts down on the pile of junk at the end of the trip!
We are pretty strict about spending money. It's hard for us to save up for Disney trips (I make about $12K a year), but we manage to go almost every year. DD is now 17 but buying has never been an issue. I am in charge of the budget, so I determine what is spent. I just say "no" if it's not in the budget, and she's never been one to nag or whine for something (maybe she learned at an early age that that's the BEST way to turn a 'not today' into a full, concrete "NO.") Even if she has her own money, which is rare as we don't have gift-giving relatives, I have veto power; if kids didn't need direction, guidance, and support, they wouldn't have parents or be kids, they'd be born fully-grown! This has kept the spending, and the junk pile, to a minimum. I usually buy DD ONE gift per trip, and it has to be affordable or justifiable (and not by her). The past few years, it's been pick-a-pearl, and one year there was a dress from AK (she was sweltering, the dress was $25, and I knew she'd wear it all the time, which she has) in addition to the pearl. Jeez, I sound mean and stingy! I always get her a Disney tee-shirt before we go, so she has a collection of (sanely-priced) DIsney shirts, and an autograph book and pen (although not anymore, sigh...), and we always have a snack (dole whip, mickey bar, etc) every day from my money, because I buy the food and pay for the trip! It's harder when we travel with other families and kids, and I give in more easily, but DD usually never really wants a bunch of junk. She'd rather have another nice dinner at a restaurant, or do something "special", like see Cirque du Soleil, take the Behind the Seeds tour, or go to
MNSSHP. Don't get me wrong... we do have a few stuffies, and some Disney kidsjunk jewelry, and some pierced earrings for each of us, but for the most part, DD would rather have another trip than lots of stuff from THIS trip. I guess I am pretty lucky that way. Last August, DD's souvie came from WWHP; it's a Luna Lovegood tee shirt that she's worn about a dozen times since then. I splurged and bought myself a flexible Disney cutting board for $3!