This advice is just TOO good not to pass on ... check out Sue Holfeld's advice at The Bride's Guide:
For some reason, RSVP'ing to an invitation is hard for guests. And even some guests who are prompt to send their reply cards back manage to goof.
Their only tasks are to 1) write their name(s) on the card, 2) check "yes" or "no," 3) stick it in the envelope, and 4) put it in the mailbox. But many are the brides sitting there with a reply card in their hand checked "will attend" and wondering, "Whose RSVP is this?"
Because the guest forgot to write in their names on that little line that says: M______________________.
There's a simple trick that will help: Number your guest list. Then use a pencil to write the number on the back of the reply card. Sneaky smart, right?