Ask guests to give shower hostesses a baby picture once they've sent in an RSVP to the baby shower.
This can simply be included in the wording along with your phone number or email address or it can be included as a stamped RSVP postcard that your guests can simply drop in the mail to let you know they're coming.
Use the stamps on your invites and RSVP cards to express your personality.
To save on postage, use a postcard for the RSVP or ask people to reply by telephone.
You'll want people to RSVP within enough time for you to shop for the event.