I am continuously packing, from one trip to the next.
We have two large Rubbermaid "Disney" bins. Whenever we return from a trip, many items (Disney visors/hats, messenger bag, annual passes, Tevas, matching Disney t-shirts etc.) go straight into the bins (well the shirts do after they get washed).
Also, anytime I come across anything that would be good for our next trip, I toss it right into a Disney bin.
By the time our next trip rolls around, I hardly have to hunt down anything for our trip, as most items are in the Disney bins.