Stop buying things
Just stop spending your money on junks you actually don't need and not only you help the environment but your wallet will appreciate it.
Stop using your car
Walk, cycle or use buses or trains if necessary. Once again, not only you save money but your health improves.
Do not eat more than necessary. Not only you save money and improve your health, but once again you help the environment.
This is not a joke answer. Consumption of money, petrol or food is directly related to consumption of crude oil, natural resources and energy.
The more money you spend, the more damage you cause to living environment.
Well, unfortunately I forgot to mention that not buying things will probably lead to economic crisis. This is why no proper politician will promote such drastic method.