Is there any consistent appraoch to where you should put the words 'Urgent' or 'Personal' in a letter layout?
Should it be the first thing (i.e. above the address) or after the Date?
"Personal" should be put on the envelope to ensure that the recipient opens it him/herself.
You can insert this sort of information above the recipient's name and address at the start of the letter:
Mr. Joe Bloggs,