I try my best to reuse things as often as possible. It's good for the 'Stash and good for the environment. Here are some ways to make reusing a part of your sustainable and frugal life:
Personal care products: The co-op near my house has shampoo, conditioner, and soaps in bulk. So a couple years ago I bought reusable containers for these and bring the containers to the store when they're low to refill them. Most things are cheaper in bulk than they are if you buy them pre-bottled from the store. I especially love Dr. Bronner's soap, it's a super foamy castille soap, I use it as shaving cream and as my daily soap; I've used it for washing a dog before, and according to Dr. Bronner's it has 18 uses.
Cleaning products: I make my cleaning products at home in reusable spray containers. They are much healthier than your typical cleaning products, and much cheaper than organic ones. I mainly use the all-purpose spray from this website (which you can make with Dr. Bronner's soap):
http://www.macheesmo.com/2011/01/four-homemade-cleaners/I've also started collecting my abundant orange peels to make this cleaner:
http://www.pennilessparenting.com/2010/11/homemade-orange-cleaner.htmlCleaning instruments: I cut up old t-shirts to make rags of varying sizes which are great for cleaning and for blowing your nose. I have a mop that has a microfiber head that can be washed to save on spendy mop-head replacements.
Food containers: I reuse all sorts of food containers. My favorite water glasses are mason jars. I use the plastic containers hummus comes in to store cut up vegetables. I use glass jars with lids to get food in bulk from the grocery store to cut down on waste. I also use these glass jars sometimes for making beverages portable (tea, or alcohol I'm taking over to my neighbor's). One warning on this is that reheating plastic containers (or even reusing water bottles) can cause chemicals to leak into your food/beverage and then get into you, so I do not recommend it!
Gifts and cards: I buy some of my clothing secondhand and one of the stores I go to has really pretty paper bags. I save all sorts of attractive paper that I get for free and use these, along with cut up paper grocery bags, to make birthday/holiday cards and wrapping paper. I like to wrap presents up with string instead of tape as this is reusable and cheaper. There's also a free store near where I live that carries donated crafting supplies.
That's all I can think of right now. How do you reuse?