I usually couch my views with words like "but hey, that's just my opinion." Essentially it's to signal that I'm not emotionally invested in some never ending argument. If people persist I say "I've said my piece, you know where I stand." Or to make them feel better, "well, who knows, maybe you're right." The key is simply not getting angry or emotional as some of these yahoos tend to be when discussing any issue.
As for your larger question...I think what people said above about having a higher purpose is key. It gives you inner peace and leaves you less consumed with keeping up with the Joneses. These people are often sick with consumerism and a lack of purpose themselves. I've noticed since I left the USA I've seen it less. It still exists, but where I live people are more into the simple day-to-day satisfactions of family, friends, nature, good food, music and dancing, etc. A car is a thing that gets you to work and a TV is what you watch the ballgame on. Sounds cliche, I know. A job can be an ambition but it's not an identity.
Personally I tend to deal with consumerists politely but with honesty. "Good for you, man. I haven't owned a car in years, and I don't even know which brands are any good. But it sounds like you made the right choice." Or, "You fellas enjoy your ****tails, I'm having a Guinness." If you want to be more direct, when they brag you can always say "that's terrific. You must be very happy." If they are, they are.
