To prevent the puff pastry from becoming soggy, make sure to slice the apples thinly and remove excess moisture. Pat the apple slices dry with a paper towel before arranging them on the pastry. Additionally, avoid overloading the pastry with too many apple slices, as excess moisture can compromise the crispness of the layers. Baking the pastries in a preheated oven also helps ensure the puff pastry cooks properly, preventing sogginess.