Help them dissociate the idea of possessions and status. The most ecological lifestyle is always the most modest one: small flat, no car, second hand everything, eat vegetarian, local holidays etc. That lifestyle, writ large, will do more to save the planet than anything else one can do, and by very far. The problem, of course, is that such an ecological lifestyle is also a lifestyle of modesty - and modesty, when one is young and the competition for access to reproductive resources at its peak (yes, I'm a biologist), is social death. Thus, anyone who can solve this conundrum will, in my book, be as transformative as Haber, Pasteur or Einstein and deserving of the similar honours.
Offering a reward in the short term. Today young people are accustomed to immediacy (instant messaging, high-speed internet, 24h post-mailings, etc.). There could be offer money or something for X plastic bottles, etc. I know that in some countries there are already such policies, but not in all of the EU (in Spain not, for example). And trying to fashion the fact of caring for the environment, making it a lifestyle and not something that is an effort / boring work.
Set benefits for such lifestyle: discounts at shops, free entrances for entertainments.
This so hard nowadays...But if ı share solutions; Read so much and understand the world's order. For example; producing your own foods in your garden is million times better than comsuming the genetically modified foods from shopping centers. Also when you are ill, using the endemic herbs much better than using pills. But as we can see ,ıf young people have ecological lifestyle ,they wont spend a lot of money.Our world's system will never let it to be...
Educate them in school, share it on facebook, make it a trend.