i wouldnt right the letter, It would create a nasty time paradox. EXAMPLE:
I write the letter saying: "dont get that job as a waiter as it will drop your confidence to a level bodering on suicidal" - Then post it to past me, Then i wouldn get the job, therefore changing me in the future, therefore i wouldnt write the letter, THerefore i wouldnt know to NOT get the job...
Ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!