What is the secret of making a really juicy burger?
Use only good beef, salt and pepper. The beef should be a good fatty cut, with a ratio of 80% meat, 20% fat. Chuck is ideal. Any good butcher should grind it to order for you.
You need to keep the ground meat as cold as possible, to prevent the fat melting out of it before you cook it. Do not salt the beef prior to shaping, just use pepper. Shape your burgers, then put them in the fridge for a couple of hours.
Generously salt the outside of the burgers about 1 hour before cooking.
Get a good pan (preferably a cast iron griddle) really, really, really hot. Brush one side of the burger with vegetable oil, then slap it on the grill. DO NOT PRESS IT WITH THE SPATULA, EVER. You squeeze out all the juice.
Turn the burger and cook the other side. Timing will depend on how you like your burger. A good way of testing doneness is to insert a thin metal skewer into the centre of the burger from one side (not the top), leave it for a few seconds, then take it out and touch it carefully to your lip. If it's hot, it's just about done. Or if you have a good digital thermometer, use that.