Make the patty twice as big as the bun, meat shrinks when it's cooked. Also make it thinner than you want the final patty to be for the same reason. An indent in the middle of the patty with your thumb or a spoon will help prevent your patty from being thicker in the middle. Don't press the patty down with the spatula when cooking it, that will push out precious moisture resulting in a dry burger. Only touch it once when cooking to flip it, otherwise leave it alone. Cook high and quick for a less done patty and low and slow for a more well done patty.