How can I keep sliced pears and apples from browning on a fruit platter?

Like Amanda says. I keep a spray bottle with about 6oz of water and 2oz of fresh lemon juice on hand, and spray them when cut. Easier than soaking then tossing out the juice/water mixture.

I soak them in pineapple juice for a couple of hours.

I agree with Amanda.You can also mix few slices of Orange, Tangerine or Grapefruit. This will do the same trick as lemon to delay the process of brown-ing !

Toss the slices with lemon juice before laying them on the platter. This won't prevent all browning, but should delay the browning for a bit.