How can I keep fruit salad fresh longer
There is actually no way to keep fruit salads fresh for days, but at least you can make fruit salad and eat it the the next day.
At times I also make fruit salad at keep some for the next day. The biggest problem is that some fruits like apples and bananas oxidizes and turn black very quickly, while fruits like pawpaw and watermelon goes mushy when left to long. To counteract that, add a little lemon or lime juice to your fruit salad to stop fruits from oxidizing, and keep your fruit salad in the refrigerator to keep it from going mushy. This should keep your salad "fresh" for at least the following day
On your question which fruits are the best, there really is no exact answer to that. Freshly cut fruit don't last long, either refrigerated or covered with fresh lime/lemon juice. It still stays the best to make fruit salad and consuming it as soon as possible. And why compromise and leaving out your favorite fruits because they won't last a day or two in your salad. And making fruit salad is always a pleasure
You may consider vacuum sealing promptly after cutting before deterioration occurs. I've found that cut fruits, avocado, onions, tomatoes, etc. will maintain their freshness and basically stay unchanged when vacuumed sealed.
Proper vacuum sealing should remove all air from the package thus preventing the oxidation process that causes discoloration and deterioration.
Most important: Add your acids at time of making (inhibits oxidation), but do not mix in any syrup, sugar or salt (which will bleed a lot of water out of the fruit).
Use as sharp and thin a cutting tool as you have (no serrated knife, no worn peelers, no graters. A razor sharp, stainless, thin knife is best.) in order to keep the cutting surfaces minimal and clean
I know they say fresh pineapple is better for you, but when I make my fruit salad, which I do everyday and keep for 2-3 days, I open a large can of pineapple chunks in juice, pour into a container, add my apples and oranges, put the lid on, give a little shake to coat my fruit and just use when I want. If I then decide to add some fresh pineapple, I can.
There are no exact solutions but you can cut your fruit individually, such like keeping the apple pieces alone and banana pieces alone and put each in a plastic bag. Then get the air out of it completely. Whenever you want some you can mix them and eat them at the moment. This solution doesn't fix the problem completely as fruits can't stay fresh for so long but it would help make them last a little longer.
ps: Dont forget to keep them in the refrigerator
Sauce will help, at least for a few days. I don't think it actually matters what kind of sauce.
We tend to make fruit salad every year or so for get-togethers... a lot of fruit salad, because we need enough for a lot of people even if not everyone will take it. Usually, we use a cream sauce - creme angalis, melted ice cream, sweetened heavy cream, something of the sort.
In any case, leftovers in tupperware usually last 3-4 days, easily, and perhaps a couple more days with browning and softening, and otherwise less quality or taste.
Why does it work... I would guess that the cream sauce basically works like the vacuum seal Cindy mentioned, it displaces the air out of contact with the fruit, and even provides some limited protection (as a thin coating) for the fruits on top and most exposed to the air.
So, does it have to be cream sauce? I would guess not. Simple water might work, but you would be diluting the flavors as they osmosis'ed into the water - might work better if they were stored in water in somewhat larger chunks to minimize surface area. Juice might work, but then the whole salad would taste of that juice (may or may not be a bad thing) - might be what Jo Cooper was getting at.
Maybe milk, if you wanted a neutral-ish flavor, and use the last of the fruit (and the soaking milk) to make smoothies on the last day? And obviously various cream sauces work, since we've used them.
Lemon or lime juice, I've heard, can also work pretty well - the acids slow browning, and the flavor profile should be pretty nice and zingy, especially given some time for the flavors to mingle - though you'd need a lot of lemon/lime juice, it would get sweetened from the fruit sugars and (in the description I heard) could then be diluted into a nice fruity lemonade or something.
In short, any liquid plus a sealed container to store the combination in will minimize the air exposure that makes fruit salad brown and soften. Since your fruit will be sitting in it, a flavorful liquid could have something to add to your salad, while a mild liquid (or actually, any liquid) will also be flavored by your fruit - so it depends on what you want from the salad.
I use a sauce made with 1-1/2 cups orange juice with 1 package of sugar free vanilla pudding whisked in for 2 minutes(you can use regular pudding), then added to a large bowl of fresh fruit...blueberries, strawberries, fresh pineapple, red grapes. It stays fresh for several days in a covered container.