Avoid over-handling: Don't over mix or knead the burger mixture, as this can make the burgers tough and more likely to shrink. Just mix the ingredients together until they're combined, and then form into patties.
Don't compress: When forming burger patties, use a light touch so as not to compress the mixture too much. This will also help to keep them from shrinking during cooking.
Make a dimple: Before cooking, use your thumb or the back of a spoon to make a small indentation or dimple in the center of each patty. This will help the burgers to cook evenly and prevent them from shrinking too much.
Use a higher fat content: A higher fat content in your burger mix will help to keep the burgers moist and prevent them from shrinking too much. A mixture of 80% lean meat to 20% fat is best. Add some extra ground beef, or substitute in some ground pork or lamb for higher fat content.
Start with a cold or frozen burger patty: If you start with a cold or frozen burger patty, it will help to keep the burger from shrinking as much during cooking because the heat will not penetrate the center of the patty as quickly.