Usually sprinkling some acidic liquid on them should help, like adding lemon juice to water. Or, usually when I make a fruit salad, I'll add oranges, tangerines, grapefruit to it & squeeze some of the juice over the salad & mix well. Also, try & serve the salad shortly after making it. Or, if you have to make it well in advance, add the apples & bananas at the last minute.