Stay Hydrated - It's amazing how much water weight we loose while exercising, sleeping and traveling. We're familiar with exercise, and the airlines and others have made us more aware that we need more water due to the greater loss at higher altitudes. Every night we loose a couple of pounds while sleeping.  Forget the bottled water, except at airports, where you can purchase some after getting through security. Purchase canteens, good quality reusable bottles. You'll recoup that time you spent buying, collecting and recycling the plastic bottles.

Pack a Picnic - Avoid fast food whenever possible. It will keep you from starving, but the waste you generate will typically go straight into the trash and onto the landfill. Alternative - pack and ice chest; don't stop by  convenience stores - go to a grocery store. You can pick up fruit and deli items a lot cheaper and with less packaging, and there are many more healthy choices available.

Read your owner's section on maintenance. Make yourself an annual check list and stick to it.

Maintain your vehicle. Keep it in good shape. This will help it run at it's most efficient. This includes regular tune-ups, replacing the air and fuel filters, rotating the tires routinely, as well as keeping them properly inflated. 

Slow down, watch traffic for signs of slowdown, avoid jack rabbit starts and stops, coast whenever possible. Watch ahead for traffic signal changes. Excess weight from unnecessary items, large or small should be put elsewhere. The extra weight requires more energy to move and therefore more gas.

Plan your trips when you run errands, etc. Time your trips to avoid congestion, when possible. This will reduce your frustration as well as the impact heavy traffic has on how you drive. Other drivers' inattention can cause you to have to slam on the brakes and waste all the energy it took to get you up to that speed. 

Leave plenty of room between you and the vehicle ahead of you. This has more than one benefit. It's safer, as it gives you more time to react, and, when you see those taillights, you lay off the gas and coast for a bit. You may not even need the brakes!