The important thing here is that you connect with each other, and to allow your parents to choose the stuff they want to keep to make them feel comfortable with the idea of moving out. Remember many of these items are part of their life stories and not including them would be wrong.
Work slowly. Keep in mind that you should not rush in cleaning your folks home. Even though you are so eager to throw away their junk in just one go, you should give them ample time to sort through the stacks of postcards, fridge magnets, kitchen tools, and other trinkets they have.