It's kind of sad that we stop talking about alternative forms of energy when gas prices are down. A few years ago when gas prices were in the $4 range, there was always a story about how to reduce it, how to buy a hybrid, etc. People complained. Hummers stopped being popular. Hybrids were the thing. Everyone called for better mileage. Now, not everyone cares so much, but this shouldn't stop us. There are millions of cars available today and you have the choice and the power to decide what is best for you, but what is also best for the environment.
If you don't "need" a car, perhaps not getting one is the smartest choice. I realize there are conveniences to having your own vehicle, but if it only gets used on the weekends and if you have decent public transportation, biking or walking options, it may be a good idea to not have one. You can reduce your impact and save yourself a monthly payment, insurance, maintenance fees, parking tickets and worry.
If you need a car, check out those that have good mileage. You can check out the EPA's site for Green Vehicles. Green Vehicles can be those that have excellent fuel economy, those that are powered by solar/electric or biodiesel. Check out car manufacturers that also build with minimal waste guidelines. Subaru has been a standout brand in reducing their waste during the manufacturing process. Full disclosure that we have a Subaru, though not a hybrid, but has exceptional gas mileage which means we also save money on gas.
If you have an existing car already, take care of it. The better you maintain it, the longer it will last. Maintaining it well also ensures that you don't get surprised with emergency bills leaving you with less money in the bank. Keeping your car longer also reduces waste as nothing new needs to be manufactured. Remember that each time you lease a car, a new one had to be manufactured. Each lease also comes with it's own costs. Do not get sucked in the marketing that you need a new car every few years. Many of us have been taught that a car payment is the way of life, but it doesn't have to be that way.
One thing that you can also do is to put away money that you are saving from gas because of better mileage. When gas prices were in the $2.20 range, my husband and I calculated how much we were saving per month when compared to it's height of around $3.00. Then on a monthly basis, we automatically deducted around $40-50 a month of savings from gas alone. This went on for a few months as gas prices remained low and we ended up with a few hundred dollars later that we used to visit family.