How Do You Turn Down an Invitation?

Have you noticed that sometimes the most difficult thing to do is say "no" to someone? This point was raised in the comments on a post regarding the problem with getting guests to RSVP. "J" writes:

"I’d like to present another view that nobody has mentioned. The RSVP tends to invoke feelings of guilt when you respond "No"... and many people may not respond simply because they don’t want to be grilled about why they cannot attend.

...some people are introverts and simply do not enjoy gatherings and public events and would rather avoid those scenes. It’s not done out of any dislike of the host/hostess, but rather the terrible unease, and in some cases, anxiety, that a social event can produce. How exactly does someone like that politely say "No thank you," when they don’t really have an “excuse” for not attending (at least not one that an extrovert would understand)?"

The same question has also been raised in our Forum recently. This reader has received a few responses so far, but I'm curious to hear how other readers handle this situation, especially if, as "J" says above, his "no" response is due to social anxiety, or in the case of the Forum reader, it's to save on expenses. Feel free to post your response in the comments below or in the Forum.