I normally lay out the suitcase about two months ahead of time and "prepack" things that I know I won't be needing until then. Things such as swimming trunks, shorts, tanktops, wdwmagic T-shirts, beach towels, etc. Over the course of the next 7 or 8 weeks leading up to the trip, anything I think of that I will need, I'll throw back there in the extra bedroom with the prepack stuff. I don't think I've ever forgotten anything...I know I shouldn't with all that time to add stuff to the pile before it comes time to pack the final necessities the night before.
Now I don't know what I'm gonna do next time we go, since our son (who is now 2 1/2) will be moving out of the nursery and into that extra bedroom before our next trip. I don't know where in the world I'm gonna do our prepack. The nursery is out because the new baby will be in there. So I may just have to become a normal person and do all our packing the night before from now on. :lol: