You've gotten a lot of good answers, but I just wanted to say - I've thrown a lot of showers in the past and I've had much more success with putting:
Regrets Only: *Phone Number*
You'll tend to get fewer phone calls from the people who can't make it, and the ones who can don't have to go out of their way to let you know :).
Last time I put RSVP, I got five phone calls and had 22 people show. When I threw a shower two months ago, I put Regrets Only, got three calls and four people didn't show.