Designing, stuffing, mailing ... oh, my. The last thing you want to do when you start receiving replies back is worrying about who's reply is who's. The ever faithful M____________ line is where your guests should write in their names ... or provide a return address on the back flap of the reply envelope. But this is not always the case. One quick step can save you hours worth of detective work.
Set yourself up a spreadsheet or word-processing document numbering all of your addresses - then simply number (somewhere inconspicuous - like the back corner) all your reply cards - making sure to carefully match up the corresponding number to the corresponding address when it comes time to mail. So that when Uncle Fred & Aunt Sally forget to write in their names and supply a return address on the reply envelope - you know to check them off the list. Hello! Genius.