ETA: Do you babywear? I highly recommend bringing a baby carrier. This will make it a lot easier on you when she wants/needs to be held. I did this when they were 2.5, my father and I wore them for maybe 3 hours over the total trip. Not a lot, but we were very happy we had the carriers.
1. As PP said, skip the lines. Plan ahead so you know the best times to go to the best parks on what days and tailor for FPs to the child. IMHO, a happy child is a happy trip. When I took my twins at age 2.5 and 3.5 I knew what to expect of them, I knew that if that wait time said over 20 mins, we weren't getting in that line. We did go in a low crowd time of the year but it worked great.
2. Know that you will not be able to bring a stroller through the lines. I sooo wish they could just redesign all the queues so that you don't park your stroller til the bitter end! (highly unrealistic but a girl can dream). I did not know on our first trip that I would have to abandon the stroller before getting in the queue, and not always right outside of the attraction.
3. If she is a sitter, as mine are, I said screw it and let them sit at my feet in standstill queues, ones where you wouldn't move for 3+ minutes. Because even sticking to our 20 minute wait rule, a few times we ended up waiting longer. I know the ground is disgusting, frankly anything they touch there will be germ filled. They never got sick and they popped right up whenever it was time to walk forward so we did not slow down the line.
4. $1 coloring packs, such as those found at Target. These are a lifesaver! I buy at least 2 per kid per day. They come in re-sealable bags, with 4 crayons, a page of stickers, and a mini coloring book.
5. Lollipops. Kid getting cranky? Plug that mouth with a sucker!
6. When all else fails, hand over your phone and find their favorite TV show on youtube.
I know others do not share this philosophy of mine but when the kids get cranky & tired at Disney, I happily pacify them with sugar and screens so that we can go about our merry way. For us, we will never go to Disney more than once a year, and we always take short trips no longer than 4 days. It never leaked over into bad behavior once we returned home.
I will be holding them to a little bit of a higher standard this time as they will be turning 5 during our trip and are old enough to understand "if you want to ride this ride, we will have to stand/walk in this line for the length of 2 episodes of jake and the neverland pirates".