Chaz, topping migration towards the middle, on NY style pies, is almost always a sign of improper edge stretching resulting in a bowl shaped skin.
Edge stretching is something that very few beginning pizzamakers incorporate. They generally go straight from the finger press to the knuckle stretch. If you want the ideal NY shape to the skin/no bowling, it's critical you do an edge stretch. Take a look at
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There's no more effective way, imo, to ensure that the thickness at the tip is identical to the thickness close to the rim. It feels incredibly awkward when you first try it, and, even after doing it for about two years, it still feels a bit awkward for me, but you eventually develop a rhythm. It's almost like the children's game of rubbing your belly in a circle and patting your head- it's hard to do because your hands are doing such different tasks.
Other factors that prevent bowling are making sure you knuckle stretch only the edges and nowhere near the center, as well as pressing out a slight mound in the middle when you're doing the initial finger tip press.
Even on a properly stretched, non bowled pizza, you will find some migration, although it's not always towards the middle. You can help counteract this by pressing out a smaller rim as well as by using the right consistency and quantity of sauce. Wetter sauces and larger sauce quantities promote migration.