I always have odds and ends of citrus sitting on the counter or mouldering away in the back of the fridge. Dave will peel two strips of zest from a lemon for a negroni and the poor thing—I swear on a stack of Maida Heatter's books I had every intention of using it—will gradually wither from embarrassment and end up in the compost bucket.
One day it struck me that I was wasting a heck of a lot of perfectly fine fruit juice, not to mention zest, that could be used in cocktails, desserts, salad dressings and any number of other recipes. You know the ones, where they call for a teaspoon of juice or a pinch of zest or a wedge for garnish, only requiring a portion of the whole.
That's why I've taken to first zesting the fruit with a microplane grater, which shaves off only the desirable outer layer and none of the tends-to-be-bitter pith, then juicing the whole thing no matter how much a recipe calls for. What juice I don't use is poured into a container, labeled—an important step, believe me—then stored in the freezer. (An ice cube tray would work, too, and the cubes can be kept in a container or zip-lock bag. Zest can be stored in zip-lock bags or those teeny lidded containers that have no other use that I can discern.)
I can't tell you how many times we're short a mere half-teaspoon of juice for a recipe, or we'll need zest for orange currant scones, or Dave says, "Dang, I'd make a daiquiri if we had some lime juice." That's when I pipe up in my most helpful tone and say, "Hang on, I think there might be some right here…"