There really is a trick to packing bags of this type. Everyone has there own method but here are some of my suggestions.
Go to WDWIG.com she has a great packing list!!!
1. Pack a change of clothes in your carry on just in case. Also swimsuits and meds.
2. Put socks and underwear in ziploc bags (gallon size work good or two gallon if you can find them). Squeeze all the air out of them. It really shrinks down the "bulk" of these items.
3. Roll your clothes. Folding will only give you lots of wrinkles and rolling takes less room. Roll outfits together if you want or Shirts inside each other.
4. Remember the bottom of your suit case is where the wheels are. Pack shoes and heavy items down there. Helps to prevent the suitcase from being top heavy.
5. Don't know if you are bringing kids, but I do the ziploc bag thing for DD also. Each outfit including sox and underwear are all placed in a bag and zipped closed. It helps in getting dressed in the morning. everything is all together and we don't have to search for stuff. (Also prevents DH from putting the orange top with the pink shorts and the lime green sox!!)
6. Oh Yeah besides your extra clothes in the carry on, make sure you put at least one set of everyones clothes in each piece of luggage. If some of it gets lost you at least have a chance of having some clothes to change into.
FINALLY!!! do one of two things, Pack your bags VERY loosely or pack an extra bag. Who wants to sit there and repack on your last night there. It is so much easier to just pick up the clothes and dump them into the suitcase, then run for your last chance in your fave ride!!! You also need room for all the Souvies you are going to bring home!!! Put those in your carry on! I assume you aren't going to pack ALL your clothes so you don't need to worry about lost luggage on the way home!
Have fun