Check out this website: http://www.pictureitpostage.com/. Click the "Get Inspired" tab and head to the Wedding Section. You can customize your stamp to match the theme of your wedding stationary! If you're doing a themed Save The Date, include a coordinating graphic. On your Thank You Notes, include a wedding photo! If nothing else, pick something plain that doesn't clash with that great first impression your invitation is making.
If custom stamps aren't in your budget, head over to http://www.usps.com/ and choose the Wedding Heart stamps...anything to avoid that quail!