When you get your berries home, prepare a mixture of one part vinegar (white or apple cider probably work best) and ten parts water. Dump the berries into the mixture and swirl around. Drain, rinse if you want (though the mixture is so diluted I find you can't taste the vinegar,) and pop in the fridge. The vinegar kills any mold spores and other bacteria that might be on the fruit. Raspberries will last a week or more, and strawberries almost two weeks without getting moldy and soft.