1. Soak them in a bowl of cold salt water for 3-5 minutes. Not too much salt, or it effects the taste of the fruit. Dissolve 1/2 teaspoon of salt per 1 quart (4 cups) of water.
2. Soak them in a bowl of cold lemon water for 3-5 minutes. This is better than squeezing the lemon juice directly on the fruit, unless you don't mind the fruit tasting really lemony. Mix 1 tablespoon of lemon juice per 1 cup of water.
3. Soak them in a bowl of Sprite, 7-Up, or a similar lemon-lime, carbonated soda for 3-5 minutes. I don't know if there's really citrus in these sodas, and I don't know why this works; but it does.
4. Sprinkle them with Fruit Fresh. This is a citric acid powder that most stores have with their canning supplies.
I'm more likely to use the salt water or lemon water methods, because I have those ingredients on hand. A quick soak, and your apple or pear slices are ready to store in the fridge.
Store the fruit slices in a ziploc bag.After soaking my fruit slices, I drain off the liquid and store them in a ziploc bag in the fridge. Squeeze as much air out of the bag as possible before sealing it. Amazing how long the apples stay fresh and and crispy this way. Their storage life can vary depending on the type and freshness of the apples. If you're using apples that are in season and fairly freshly picked, they will stay fresh longer than those that have been picked and stored for a long time. I've had them keep for up to a week.