I make a very detailed packing list and check off each item once it goes into the suitcase. Each suitcase is then carried to the front door where it remains until the shuttle service picks us up.
I know exactly how many suitcases we are bringing (I pack a lot) and do a quick count at the door, at the curb, at the airport check in, as we board with our carry-ons, etc., etc. Each person in our family knows how many bags he is supposed to have and how many carry-ons.
In all the hussle between point A & B, it's so easy to forget and leave something behind. Your mind is on 30 different things at once and before you know it, your minus something. The quick count really helps us!