I don't have a Disney box, so much as a "vacation bag". It's a beach bag I have filled with swimsuits and cover-ups because I rarely, if ever, use these at home. When I know I'm going on a trip, I pull out that bag and start adding to it. We have had our Disney trip planned for 7 months and we now leave in 5 weeks and I'm sure I'm still forgetting things! I started collecting things little by little and putting them in the bag. I'd rather spend $5 here and there for things I need than run around a store like a mad woman finding stuff I need the night before LOL. I also pack my luggage little by little over the course of a week and just add in the last couple items the night before and morning of.
I have a packing list & cross stuff off as soon as I put it in the Disney box! My boys think I'm crazy 
We're going in September and we also have a large beach bag that's full of swimsuits, t-shirts I bought from