We have found that taking a break in the afternoon and going back to the hotel prevents meltdowns and actually gives us more time in the evenings. Even if your little one isn't taking a nap, it's nice to go back for a couple hours and chill out, away from the crowds and hustle and stimulation. So a little bit after lunch, we head back to the hotel. This is another great reason to stay on site- it's easy to do.
There are Baby Care Centers at every park (check the map- they are marked. I think it's a pacifier.) They have a nice changing area, a tv, microwave, bathroom facilities, a little store with kid items that you might have forgotten.
Go at your child's pace. If they want to ride a ride again after they just got off, go ahead and do it. We let our daughter be the decision maker with some guidance from us. We went to Disneyland last month, and this meant that we rode Small World half a dozen times, but it's what made her happy.
I bring some of our own snacks- you can spend a fortune in snacks. While it's nice to buy a special snack (we're churro fans), it's not nice to buy a snack several times a day.
Have something to do in long lines, even if it's a game of I Spy or 20 Questions.
We always bring our own stroller since it's more comfortable than the Disneyworld ones (the Disneyland ones are quite nice), but everyone is different with that.
I can't recommend character dining enough- it is a fantastic experience! If you can do one of these, I highly recommend it. I have pics on our blog of our last one at Disneyland. At Disneyworld, we have done Minnie's breakfast at Cape May Cafe, but there's some other really good ones (the boards I mentioned are excellent help with that).
I'm trying to think of others, but I'm blanking right now. :) If I think of more, I'll stop back.
I also really love the boards are disboards.com for tips.