For Credit Cards:

• Be cautious about disclosing your
  account number over the phone
  unless you know you are dealing
  with a reputable company.
  
  
• Never put your account number
  on the outside of an envelope or
  on a postcard.
  
  
• Draw a line through blank spaces
  on charge slips above the total
  so the amount cannot be
  changed.
  
  
• Don't sign a blank charge slip.
  
  
• Tear up carbons and save your
  receipts to check against your
  monthly billing statements.