Number RSVP cardS and log who has which number on a spreadsheet. Easiest way to track who still owes you an RSVP. When you don't get them back (and there will be many that you won't, its just how it goes) you know exactly who to confirm with.
Collect the rain drops if it rains on your wedding day. It did rain on my wedding day this would have been fun to have. Everyone told us it was good luck if it rained on your wedding day. 28 years later I guess it's still working.