About 2 months out I make 4 lists. "Things to buy", "things to do", Things to pack (items under each person's name) and "things I cannot leave behind". I go over those, check items off as things are bought and done and make any changes or additions until about a week before and then pack. It took longer early on; but after a couple of trips we ended up with what I call our "Disney box" where I started keeping items so they didn't have to be purchased again (misting fans, lanyards, personal fans, autograph books (if it wasn't filled), Disney pens, summer hats, dd's miniature backpack, etc.).
For one trip we actually packed in 1 afternoon about 3 days before we left. But the children were helping and I already had everything in one area.