I am a veteran of a number of trips to the "happiest place on earth" - my DH took me long before we had the excuse of children. We go for days at a time . . .
You will get wet and top and pants may become see-through. Make sure you time those rides so that you can dry off a bit after. Cotton does not dry as quickly as most manmade fibers.
Have pockets!! Take a small enough cross body-type that you can tuck it into the little pouch on the roller coasters (California Screamin' - awesome and smooth!).
Buy the current version of the book the Unofficial Guide to Disneyland - it is just fun to read, as well as being incredibly informative. If you are analytical, you will love it; if you are a go with the flow person, just follow its advice. A little planning will make all the difference.
Get up really early and enjoy the big attractions before the crowds get there. Understand and use the Fast Pass system - it is very helpful. Stay up late - the shows and the atmosphere really are magical after dark. How do you do this? Nap in the mid afternoon if your hotel is a reasonable distance.
Buy the mouse ears - they go with everything... Seriously - they put something in the water there that makes you buy crazy stuff. Have a blast!!