But it's easier said than done...and it never fails that there will be someone who wants an invite. Or someone who is invited but wants to bring more people with them.
When people hear you're getting married, they're happy for you. They want to share in your joy, and they want to be there for the party. Keep that in mind--most people are there for the party. They'll endure your ceremony for the promise of some good food and drink and having a good time on the dance floor in nice clothes. Yes, they're genuinely happy for you, but they're happier about having a reason to party down. Who do you really want to party down with? That's your "A" list. Who do you not mind seeing you cut loose and have a good time on the best day of your life? Invite those people. As for Susie from third grade or Bob from three jobs ago or that one girl you met at your bestie's Pampered Chef party...not so much. When they ask about their invite, instead of being snarky and telling them straight up they're not invited, I offer you these words: "Due to limited space, we're keeping it very small...but we'd love to get together with you sometime after the honeymoon!" And leave it at that. They'll either get it, or they won't.