Excellent post from Nature Moms blog regarding "Helping your kids see past commercialism." I would also recommend Center for a New American Dream, which has a lot of material on this - simplifying the holidays, doing things during the summer, etc. An excellent organization.
We've mostly been working on the "power of example" model - we're very anti-corporate, the kids have very limited access to TV (we mostly watch videos) and if they're watching sports with me or something we turn off the TV during commercials. (Sports commercials are the worse - beer and cars, both sold with copious amounts of sex - but fortunately they're not really interested in it.)
There's a book now, Parenting, Inc., which talks about the sheer amount of STUFF there is for new parents to buy, $700 stoller and all that. I sometimes think we're living in an alternate reality, and maybe we are (thank goodness we don't live in New York!) but then I happened to be at a middle school for an event recently and I overheard a kid making a snarky remark about another kid buying clothes at a thrift shop. I know that's something my kids are going to face, even though we buy most of their clothes at Target.
I would also recommend showing the older kids (who can understand it) The Story of Stuff, a 20-minute video which talks clearly and pursuasively about the environmental and human impact of our (Americans') unending appetite for STUFF. That's a lesson we'd all support, I think, and it's not just dad pleading poverty this time.