With so many replies, these may have already been mentioned but....
Getting a bagllini backpack:
http://www.baggallini.com/best-sellers/messenger-bagg/MES160-BG.html#start=3
- I've taken mine to WDW 4 times now and it is AWESOME! Leaves my hands free, big enough for the essentials but small enough to take on rides. I couldn't think of doing WDW without it. BONUS: I bought it in bright ORANGE so my kids can always find me in a crowd
Mailing a package to my resort in advance. Usually I pack rain ponchos, breakfast stuff, sunscreen, and snacks and mail it off a week in advance. Saves me $ and packing space.
Having a flexible plan. For each day, I have a park in mind and within that park a couple of "must do" activities. Everything else is up for grabs, depending on how we are feeling, the weather, crowds, etc. Each time we get to the most important things but don't stress ourselves out over doing EVERYTHING.