In a series of slides the Mayo Clinic walks through advice for maintaining good posture while standing and sitting.

For standing their advice is:
  • Hold your chest high.
  • Keep your shoulders back and relaxed.
  • Pull in your abdomen and buttocks.
  • Keep your feet parallel.
  • Balance your weight evenly on both feet.

The key advice - stand up straight and stop slouching. Here's the link to the full good posture slides.