When you get your berries home:
Prepare a mixture of one part vinegar (white or apple cider work best) and ten parts water (for every cup of water, I add an overflowing tablespoon of vinegar).
Dump the berries into the mixture and swirl around. Drain, rinse if you want (though the mixture is pretty diluted and I find you can't really taste the vinegar - test one to see what you think). I like to roll mine over paper towels too, to get rid of excess water.
Why it works: The vinegar kills any mold spores and other bacteria that might be on the surface of the fruit. Raspberries will last a week or more, and strawberries will go almost two weeks without getting moldy and soft.
The best way to store tender berries is covered, in a single layer. Tupperwear works great. Sometimes I lay a paper towel down inside of a square baking dish that will fit the berries and then cover it with plastic wrap.
What to do with berries that are past their prime:
Freeze them! Wash and pat dry your berries. Place in a flat layer in a rimmed baking sheet or pan and put into the freezer. This prevents them from freezing into a giant berry block. After a few hours, transfer into a plastic freezer bag and seal well. Use for smoothies or leave out for a few hours to defrost and add to baking recipes, pancakes, or sauces.
Cut them up (this works with almost any fruit like peaches, which spoil quickly too), removing any moldy or badly bruised parts and mix it in a bowl with a few teaspoons of sugar and ta-da! Your fruit is preserved for another few days to a week! And two teaspoons of sugar (I like to use rapadura or another natural variety) per pound* of fruit is pretty negligible, calorie-wise.Totally worth it. *Two green baskets or one large brick-sized container of strawberries is about a pound.