NotUrsula - what a wonderful rule - I'm stealing it.
Keep in mind that you can ship back stuff (laundry, souvieners) and even ship down stuff to be at your hotel when you arrive. Also, except for underwear, as NotUrsula said nearly everything you will need can be bought (at somewhat steeper prices unless you go offsite - and if you go offsite, you can get the underwear). It is, after all, Orlando, not the deepest reaches of the
Amazon.
I always plan on wearing each pair of pants/shorts twice without washing. Shirts get changed daily. But if you are going for a week, you only need to pack 3 pants/shorts, five shirts, undies, socks and a nightshirt. I wear jeans and my comfy tennis shoes. I KNOW I'll be buying at least one shirt there, so I subtract one from my total. So far, no one has ever moved from sitting next to me on the bus - so I must be doing ok.
If you are going at a particularly hot or cool time of year, the packing list changes a little - hot weather, pack some extra and/or plan on washing - too sweaty even for my low hygiene standards. Cool weather and you'll be moving from t shirts to jackets and pants to shorts throughout the day. I just bring a lightweight jacket.
When traveling with kids, I don't bring extra shoes for me (prefering extra shoes for them) or clothes for dress up. Going out dressed up is fun when its hubby and me - don't need to waste the room when packing with kids. I don't bother with makeup on vacation at Disney