Keep related fruits together
I like to put berries with berries, melons with melons, stone fruit with stone fruit. This "like with like" approach might be surprising -- contrast and variety is usually a good thing -- but with fruit salads, choosing similar fruits makes a more texturally appealing dessert. When you start tossing lots of very diverse fruits together, their textures tend to blur (think crisp apple tossed with ripe banana) and you lose the sense of each individual fruit's character.
Within one category of fruit, there are usually enough options to keep the flavors interesting. In the berry family, you've got strawberries, blueberries, raspberries, and blackberries. There are half a dozen or more melons to choose from for a melon salad; stone fruits include peaches, nectarines, apricots, pluots, and plums (plus specific varieties, such as red plums, black plums, Italian plums). You could make a tropical fruit salad with a variety of papayas and mangos, or one with pineapple, kiwi, and starfruit.