When you receive an RSVP in the mail, whether it's for a co-worker's wedding or a cousin's wedding shower, it generally is understood that a yes-or-no response is respected of you. If for some reason you cannot make the party or event, the proper etiquette is always to write a declining note. Inform the host that you cannot attend the event politely, thoughtfully and promptly.
Reply in the expected manner. The majority of invitations are equipped with response cards that you can send back to the hosts. Write out your response to the invitation on the card instead of calling or e-mailing back your host, which can make the situation rather confusing. The host may simply want written proof of guests' status, so take the time to respond as desired. If a response card is not included, respond back with a standard letter.
Thank the host. The first thing you need to do when writing an RSVP is to graciously thank the person who invited you. Even if you are not able to go to the event, show the host that you are grateful that he thought of you when making out the invitations.
Indicate why you are not able to attend. Show as much honesty as you can, because you don't want to get "caught" doing something else by a mutual acquaintance on the day or evening of the event. Whether you are supposed to attend another function on the same date or are going to be out of town on business, concisely and briefly explain why you cannot make it.
End your response positively. After you inform the host that you regret being unable to make the event, try to finish things off in an upbeat and friendly way. For example, say something like, "Had the event been for just one day sooner, I'd be able to make it," or "Perhaps we can get together sometime soon to make up for this, and I can bake you some chocolate chip cookies." The small gesture can go a long way by establishing feelings of warmth even though you are declining the invitation.
Show timeliness. When someone sends you an RSVP, it is a sign that they want to get the attendees in order as soon as possible. Be polite and avoid taking your time when it comes to responding to RSVPs. If at all possible, try to send your response back within 10 days of receiving the invitation.
To close the RSVP off, consider signing it with "Regretfully" instead of the standard "Yours truly," "Sincerely" or "With regards."