HOW TO AVOID THIS
I never realized how important an RSVP card is to a bride-to-be until I became one. Realizing that I myself could not remember sending back an RSVP card (I usually lose them with the rest of the mail), I had the feeling that other people could very easily underestimate its importance as well.
First of all, you need to include postage on the reply envelope. That is the most polite but obvious way to get the card back. People will feel obligated because you've invested in the postage. Of course, they may not be mindful of the thousand dollars you just spent on custom, letterpress invitations with matching envelopes. They're going to think they're beautiful, but people don't seem to grasp the cost of invitations in bulk. They see dollars and cents. Literally. So if you've got a stamp sitting there, most people are going to feel obligated to send it back just because it's a waste of a stamp if they don't.
The RSVP card needs to have a deadline on it, and it should be easy to remember, like the first or the fifteenth. Sort of like a billing cycle, they're likely to remember the beginning, end, or middle of the month. That way, God forbid , if they do lose the card, they can remember generally when they need to notify you. In that case, I also suggest that you get about 10 extras printed per 100 guests. If a guest calls or emails you to let you know yay or nay, you can fill out a card for them and put it in your stack. This keeps them all in one place so that you can just keep track of one pile. I would also back that pile up on a computer in case it gets lost because I'm pretty sure you'd be screwed if it did. Can't help you there.