A crucial part of any wedding is knowing how many guests will be attending. Here are a few suggestions to help make getting those responses a walk in the park.
INCLUDE STAMPS - Trust me, I understand trying to save money, but make sure to budget for stamps on your RSVP's. Not everyone has stamps lying around on a regular basis. Heck, I design snail mail for a living and rarely have them. Including a stamp on your response envelope is a must in my opinion.
GIVE OPTIONS - It may seem like a no brainer that a guest would respond when they have a pre addressed RSVP with a stamp already on it, but you'd be surprised. An easy fix for this problem is to include a wedding website to RSVP on or list a custom email address. You can set up an email specifically for your wedding using a free source like Gmail or Yahoo (doing it separate from your personal email will help keep everything "wedding" in order). Giving your guests options definitely helps cut down the number of phone calls later on.
NUMBER ATTENDING - Requesting the number attending isn't a must, but it does help you finalize your head count a little easier. If you aren't solely inviting couples to your wedding consider adding a place to list the "number attending." Or if you get a discount on dinners for children, you could add a place for "children attending."
TIMING - There is a special balance on timing for getting RSVP's returned. Give a guest too much time and they may push that response card to the side and completely forget. Don't give them enough and they can't come because they're busy already. My suggested time frame for sending out invitations is 8-10 weeks in advance. Most wedding vendors need a head count of guests about two weeks prior to the wedding date, so having an RSVP deadline of 3-4 weeks prior gives you ample time to hunt down those stragglers and get your seating charts ready.
Basically you want to cover all your bases in the beginning to make sure the time spent leading up to your wedding day is as stress free as possible. When meeting with my couples, I make sure to go over all the little details that come with such a simple piece of the invitation puzzle.