Number your RSVP cards so each one corresponds to a guest. So if someone forgets to put their name or writes illegibly, you will still know who it belongs to.
Make an email account specifically for wedding planning. This will help you stay organized and will keep wedding-related emails from getting lost in your inbox. It will also prevent you from being spammed for the rest of your life from all kinds of wedding vendors.
Use water + a foam brush to quickly (and painlessly) seal envelopes (so.many.invitations.)
Choose a postcard format for your thank you notes and cut out the envelope on the second go round.
Use a personalized return address stamp or stickers. It will save you infinite amounts of time and also you can continue to use it after the wedding!