Reusable snack bags: Buy three or four, and when folded, they take up less space than a box of throw-aways. We wash ours once a week depending on what we're carrying (almonds and crackers aren't too dirty; cinnamon-raisin bread requires we wash a little sooner). Commercial snack bags are available, like these from Plum Creek Mercantile, but so are unique (and adorable) handmade ones.
Reusable grocery sacks: To save on space, forgo canvas bags, which can get bulky, and large bags that don't fold into themselves. Instead, turn to pocket-sized bags like the ones from Flip & Tumble ($7) made of ripstop nylon: they're durable, squishable, strong, and come in a variety of colors. We keep a few of the more rigid reusable shopping bags on hand, but turn to our ripstop nylon ones most often because they're easy to toss into our bag before heading out the door.
Leftover food storage: While we love our glass to-go containers with leak-proof lids, they do take up a fair bit of cabinet space (that is, when they're not all in use). If you have enough room, look for glass containers that could potentially double as baking dishes. They can go from oven to fridge or freezer and then straight to the microwave, making them multi-taskers. Don't have room for glass? Try BPA-free plastic containers that stack inside one another. Some brands even have lids that snap into each other, making them stack efficiently.
Water bottles and reusable coffee cups: We devote just enough shelf space to keep two BPA-free water bottles and two reusable coffee cups on hand. One is always with us, the other standing by for the following day. Our rule at home is to never have more than two of those rotating items, which saves on space and keeps us washing them every day (no smelly coffee cups here!).
(Image: Marcia Prentice/Apartment Therapy)