If you use something often, find a way to use a reusable item instead. Buying the same thing again and again wastes money you could be saving and investing.
Water Mug/Container
If your office/school/place you go everyday has a water cooler or a water fountain, then do not buy bottled water. Buy some sort of mug or container that you like, and fill it up for free. You can save a hundred dollars or more a year depending on how much you drink. And I hate to be the one to break this to you, but it's the same water.
Lunch BagsIf you make your own lunch and bring it to work or school, you are already saving hundreds of dollars a year; good for you. However, if you are using disposable lunch bags and zip-locks, you are also throwing away money.
A brown bag is just a waste. Get a nice permanent lunch bag and you save money for the rest of your life. Also, instead of zip-lock bags, you can buy reusable sandwich bags. I got some and they're pretty neat. You just rinse them off and let them dry over night, and they are ready to use the next day. And if you actually make something better than a sandwich, invest in some nice plastic boxes.
Batteries
This goes without saying; Don't ever buy batteries! Get a good set of rechargeable batteries, and recharge them.
USB Stick
Don't waste money burning data on to a CD or DVD every time you just want to bring something with you to work. If you aren't going to keep that data around, you're hurting the environment, and your wallet buy using permanent media. USB sticks are cheap, small, and convenient. Get one, and use it to bring digital data with you.
Digital Camera
First, don't buy disposable cameras. Second, if you still have a camera that takes film, you better be a photographer. If you want to save money, you should buy a digital camera. Not only will you save money on film, but you will also save on prints. You no longer have to waste money on an entire roll if you just want a couple of photos printed. But even better, you can email your pictures, or use a free online service like Flickr or Picasa and save even more.
Towel
How often do you wipe up a little spot of water with paper towels, or dry some dishes with paper towels, etc.? You might as well dry them with dollar bills. Using paper towels is so wasteful. How did that become so prevalent in our kitchens. A good hand towel for drying dishes and water spills will save you money, and helps the environment. Paper towels have their uses (tomato sauce comes to mind), but not for that small spot of water next to the sink.
If your goal is to save money, then don't waste money buying things you could be reusing. While you are buying bottled water and batteries, I'm investing in stocks and making money.