Buy less stuff
Fix broken items instead of replacing them.
If you only need it once, borrow it. Books from the library, video rentals, your neighbor's power washer. It's all good.
Take a shopping list - avoid impulse purchases and buying more food than you can use before it spoils.
Save on packaging by buying in bulk - but no more than you will use.
Create your own wrapping paper or use wrapping (bags and boxes) multiple times.
Avoid disposable food service items like plates and cups, and hygiene products like disposable razors.
Use rechargeable batteries.
Carpool, or save all your car-related errands up for one trip to reduce gas purchases.
Use products that do double duty. For example: chlorine bleach removes toilet bowl stains as well as any product designed only for that use and you probably already have it in your laundry room. Baking soda and vinegar provide the base ingredients for a variety of safer household cleaners in addition to their uses in cooking.