Have you made your ADR's yet? You definitely want to do that 180 days out or as soon as possible if you are closer than 180.
There are tons of lists out there to use for what to pack--here on the DIS and on allears.net are some of the best. Here's one to start you off from all ears:
http://allears.net/pl/forgot.htm
About a month before our trip I set out a plastic storage bin in our bedroom in the corner. I randomly think of things that I need to remember to take but don't always have time to write them on my list. So to remedy that, I just toss the items into the bin so that way when I do my official packing I won't forget it! I use this for random items (like band aids or baby tylenol) not for normal items that I would pack like toiletries.
Also, another good item to bring (especially with young children) is ziplock bags--I take a variety of sizes: snack size, sandwhich size and gallon size. I use the smaller ones to save leftover food from a meal for a snack later on. The gallon ones come in handy in case one of your kids has an accident and needs to put on clean clothes--need to have a place to put the dirty ones!

The gallon bags are also good for keeping your valuables from getting wet. For that matter, don't forget to take a change of clothes with you to the parks!