It's a classic backyard snafu: You flip the patties on the grill, and suddenly what seemed like perfectly bun-wide servings of meat contract into domed pucks. This is the dreaded meatball effect, which is the result of too much shrinkage: Proteins in the ground beef coil up during cooking, squeezing out moisture as the patty tightens into a ball. Adding more beef isn't an option for health-minded cooks, and it won't work anyway. And flattening the patties with a spatula as they grill—a technique we've witnessed many times—simply pushes out delicious juices, causing flare-ups and sooty, dry burgers.
The solution: Make the raw, 4-ounce patties a bit larger in circumference than the hamburger buns. Press the mixture together gently; overworking it increases contraction and makes the cooked meat dense and dry. Use your thumb to make a nickel-sized indentation, a little more than ¼ inch deep, in the center of each patty. This prevents the burger from doming into a ball, keeping it flat and even as it cooks. Voilà: no meatballs in sight.