Depends on age. When they are really little, most meltdowns are just a response to being "too" something: too hot, too tired, too hungry, too scared, too overwhelmed. Getting out of the parks usually fixes that. We often got out to one of the nearby hotels if all we needed was a short break in a quiet place.
For older kids, the most important thing to watch out for, other than the environmental factors listed above, is the gimmies. Give each child a pre-set spending limit for the entire trip, then restrict shopping to the final hour in a given park. I create scrip for DS' spending money, and if he willfully misbehaves, I destroy it a little at a time. No scrip left means no souvies at all, and he knows it.
We visit lots and lots of museums and children's attractions year-round, so he knows what behaviour is and is not acceptable in such a place. Generally, he's much better behaved there than at home, because he doesn't want to the lose the privilege of being there.
One thing I would caution about if discipline is an issue for your children: don't buy UHP's. There is a nasty psychological barrier to those "use-it-or-lose-it" park passes, because parents tend to be unable to say "we will just stay in our room with no TV" when they are out the money for the passes even if they don't go to the parks. With a park hopper, you can follow through on that threat without losing money.