Katie from FoodLush blog says this:
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 surface of the fruit, and voila! Raspberries will last a week or more, and I've had 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.
I tried this with a 2 lb. package of blueberries that I had purchased. I actually did it about 2-3 days AFTER I bought the berries, and it STILL worked! Those are the berries, up there in that photo...being "dunked" in the vinegar water. That was on August 15th. We ate the last of the berries (of which about 99% of them were still yummy and plump!) on Monday night, August 22nd! Imagine berries lasting for over a week in your fridge! The things you can learn on Pinterest!