Buying Berries

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 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 surface of the fruit, and voila!
Raspberries will last a week or more, and strawberries go almost two weeks without getting moldy and soft.
So go forth and stock up on those pricey little gems, knowing they'll stay fresh as long as it takes you to eat them.