The why and how of RSVPs.
If I haven't conveyed yet how important it is to respond when a response is requested, it is very, very, very important. When you receive the invitation, take a few minutes to look at your calendar and make your plans. There is usually a date by which you should respond, so you don't need to rush your decision. However, responding as soon as possible will give your host a sense of confidence. The sooner she has her guest list, the more excited she'll be about the planning.
In our age of technology, there are usually a number of ways to respond. From email and cell phones to good old-fashioned postal service, no excuses should be made about not being able to respond unless you happen to be out of town. You should certainly try to respond in the fashion that is requested by the host, but don't feel constrained by that. If you just can't get the pre-stamped envelope in the mail in time, go ahead and call. And if you hate talking on the phone, just send a quick email. I'm sure the host would much rather have you in attendance than fretting over the requested means of communication.
The Divine Decline
Now, since we know that your response has ensured that your portion of trout will be available, it's important to mention that declining is another area that needs attention. Just as you responded to make your presence known, you should get back in touch with the host to let her know if you later realize that you won't be able to attend. I highly recommend sticking to your commitment, but if you are not able, please make an effort to get that information to the party planner as soon as possible.
And it's okay to decline all together. Of course your presence at the event would be appreciated -- that's why you got the invitation in the first place; however, if you know you are not able to attend, don't hold on to that information because you are afraid of hurting someone's feelings. It is far worse to not respond and be expected to attend than to decline the invitation.