This is a biggie: don’t try to throw it together in one sitting.

Make your dream list and then, due to budget or space restraints, start to cut. Ask yourself, Why do I want this person at my big day? If the answer is “because I am afraid of snubbing her” or “I was invited to their wedding four years ago,” move them to the undecided list. Family members get top billing, of course, but if many of them live out of town and probably won’t make the trip, you can probably safely invite more people who will be in attendance.

(this tip originally from Jennifer Manske Fenske)