Number RSVP cardS and log who has which number on a spreadsheet. Easiest way to track who still owes you an RSVP. When you don't get them back (and there will be many that you won't, its just how it goes) you know exactly who to confirm with.
Guests often forget to write their name on the response card. With this invisible ink pen, just number your guest list and write that corresponding number on your RSVP card. Then no one feels bad about the "number" on their RSVP card!