1. Hold your tongue - no matter how wrong they are with their assumptions or no matter how much you want to say your side, JUST. STAY. SILENT.
  2. As my theology prof said, never ever share your problems or quarrels with your spouse with them. Because you might have already forgiven each other and are all sweet and cuddly, but they are still ANGRY with either you or your spouse.
  3. They are older. So like toddlers, they can be egocentric. So always assume that they are right, and that the world revolves around them. Then see lesson #1.
  4. Never ever discuss finances with them. This includes borrowing money from them.
  5. Before saying anything, think it through 5 to 10 times over. Because what might be harmless and meaningless for you, can mean a hundred different harmful things for them.
  6. Keep your sanity intact. Do so by telling everything - your rants, problems, every little thing you can't say to their face, to your closest friends. If they have in-laws as well, better.