If you send a wedding invitation to the President, you will receive a congratulatory letter from him and the First Lady. I have to remember to do this! P.S., it works for Mickey Mouse and Queen of England too.
Hand-Canceling - Remember to do this! Take invitations to the post office and request Hand-cancelling so you don't get ugly bar-codes printed on your invitations. I didn't even know that was an option...also has other advice for invitations