Anytime I have ever been around a child who has a meltdown, he/she needs a break/rest/nap, to eat, to drink, and/or control. Of course, each child is different and my experiences are hardly all encompassing.
I think it is important to take breaks at WDW even for the child on the go-go-go at home. Going back to the hotel is a big one. Sitting down in an air conditioned attraction or taking a slow ride (train or paddle boat in MK) or getting away (Tom Sawyer's Island) can provide little ones. So can sitting in a stroller while Mom and Dad shop. I really think it is so important that little kids have a stroller at WDW, even if they *say* they don't want one or don't use one at home. WDW is much more overwhelming than a trip to the mall/zoo/Sea World/Six Flags/etc. I just always see these stressed out kids walking and I feel soooo sorry for them.
Grabbing a quick snack or sitting down (preferably in a/c) for a meal will usually do wonders also. ALWAYS have a drink on hand. We keep bottles of water on hand for the next time and not just for when needed.
And I think it is super important to follow your child's lead. If they want to ride a ride 14 times in a row, let them. Even if it means missing something else. If they want to skip an attraction, do so. Prefer to chase characters only one day, what the heck? I don't think you should try and fit a child into your schedule--or even necessarily the schedule they have at home (although for some kids that helps more). That doesn't mean if you have other children in your family they should get their wants/desires ignored. Give a child SOME choices and they'll go along most of the time. Everyone should get their turn being in charge.
AND MOST IMPORTANTLY, if your child does have a melt down... Give them a couple of minutes to regain control and if they don't, no matter what it means, remove them from the situation totally. Don't ruin someone else's meal or enjoyment of an attraction. It obviously isn't going to get better at that point, and you can always come back.