My eldest DD was 3 1/2 on our first trip to WDW, the youngest was the same age on the second trip.
Like previous posters mentioned, keep to their same sleep schedules and carry around snacks and drinks. Don't wait for her to ask for a snack or drink -- by then they're already too cranky. Keep offering while you're in lines. (Especially the drinks if it's hot. A lot of crankiness is due to dehydration and too much sun.)
My DDs go to bed promptly at 8 pm. We always made sure to be back in the hotels by that time - no late-night fireworks or parades (unless they weren't doing anything at all the next day). Generally, we knew our day was done by 5 pm.- left the park and had some dinner off-site. If we didn't see it all, there would always be another day, or another trip.
Take it easy. Plan a lot of sit-down shows. Try to get a feel ahead of time if they like characters. My eldest always did, but the youngest showed fear at characters from our local Paramount park. I prepped her with the
Disneyland sing-along DVD, and I gave her an autograph book which broke the ice with characters and gave her a mission! She was never afraid at WDW -- showed her book, and demanded a photo, and moved on
My DH has always felt children over 2 shouldn't be in strollers (don't yell at me! Everyone is different.) So, my biggest tip would be to get your DD going on long, outdoor walks to get used to the amount of walking at DL. And make sure they have comfortable running shoes rather than sandles. I couldn't believe how many kids we saw in flat flip-flops. That's got to hurt by the end of the day!
Enjoy the little things, and if they get so cranky that you're miserable, just leave the park. One of my eldest DDs fondest memories of her first trip was the great pool at our hotel