The best way to encourage guests to confirm that they will be attending your wedding is to include a response or reply card (RSVP card) with your invitation.
These standard RSVP's are supplied with 100mm square envelope which then fit into the invitation envelope.
The text can be customised to your chosen colour.
*Handy tip: Guest's may forget to write their name(s) on the RSVP card when replying, a good idea is to cross reference your RSVP cards with your guest list by numbering the back of each card.