There's nothing more disappointing than reaching for the fresh berries in your fridge only to realize that they're all covered in white fuzz. So what is the best way to store fresh berries so they don't get all gross and moldy? After all, berries can be expensive, and knowing how to keep berries fresh for longer is as much a money-saving technique as anything else. The bad news is that berries are extremely delicate, so even a little bit of mishandling can quickly ruin all of your fruit. As Harold McGee explains in On Food and Cooking: The Science And Lore Of The Kitchen, "Even rinsing in water can make delicate berries more susceptible to infection by abrading their protective epidermal layer with clinging dirt particles."
This is why you shouldn't wash all of your berries as soon as you get them home from the store. Water only exacerbates the rate at which mold will grow on your fruit, especially when your berries are just hanging out. The best strategy is to only wash as many berries as you plan on eating or using in a single sitting. And when you do wash them, don't submerge them in water. Put the berries in a colander or strainer, and gently rinse them with cold water.
If you're packing up these fresh berries, so you can take them to the office for your lunch or (in a perfect world) to the park for a picnic, place a dry paper towel at the bottom of the container before sealing it up. And be sure to keep the berries in the fridge for as long as possible, which will further extend their life.
As for the berries you're not eating, be sure to store them in the fridge in a container that can breathe. That means no air-tight plastic or glass containers—though if that's all you have on hand, just leave the lid slightly ajar. You should also place a dry paper towel at the bottom of this container, to sop up any excess moisture that might cause more damage. Megan Gordon at Kitchn insists that keeping berries in the crisper makes them go bad more quickly than if you keep them in the main compartment, noting, "In a very unscientific sampling, I find this to be true and follow this myself."
If you've given these methods a shot yet still find that your berries get moldy after a day or two, you might want to try rinsing your berries in vinegar. As our own Kat Kinsman explains, "To prevent mold growth and extend berries' freshness, rinse them in a mixture of one cup white vinegar and four cups of water, then drain and dry them thoroughly." Store them as you would unwashed berries, on top of a dry paper towel in an open container in the fridge. The idea is that this vinegar wash kills any mold spores on berries, which, in turn, prevents the fuzzy growth.
Hopefully one of these berry storage methods works for you—and if not, I would strongly recommend just eating all of the berries in your possession as soon as you get your hands on them. But really, that shouldn't be a problem.