A great burger starts long before it hits the grill. We use fresh beef with 20% fat, ground every morning, and we never pack it too tightly.
We grill on very high heat and flip only once, so the outside caramelises while the inside stays juicy. And we never press down on a burger while it cooks: that squeezes out the juice.
One minute of rest before it goes in the bun keeps the juices where they belong. Try it at home this weekend.