Communicating your intensions not only makes your ride safer, but it is also required by law in Michigan.
Left Turn: Fully extend your left arm out to the side
Right Turn: Bend your left arm up at a right angle with your hand flat (note: signaling right turns with your right hand is not yet legal in Michigan, but legisaltion is pending to change this)
Slowing or Stopping: Extend your left arm out at a right angle with your hand open
Source: League of American Bicyclists