1. Use a stud finder when looking for wall studs. Don't rely on knocking.
  2. Don't hammer a nail directly into dry wall, it will come right out.
  3. Use a wall anchor if you're hanging something that isn't on a stud.
  4. Pay attention to the weight restrictions listed on the boxes of hardware, going over them could result in your things falling off the wall.
  5. If you don't have a level handy, you can use a golf ball or a glass of water to see if a shelf is level.