If they seem "packy" then you probably are, in fact, packing them too tightly. Be gentle when you shape them -- just enough to get them to hold together.
Adding egg does nothing except give you egg in what is now more an individual meatloaf. :mad: If you're using good meat, you shouldn't have to add ANYTHING. Besides, mixing stuff in means you run the risk of handling the meat too much and compacting it when you shape it. (I'm a burger purist: I want mine to be just meat, with salt and pepper on the outside, nothing else IN the meat.) I'm not even fond of the idea of sticking an icecube in the middle -- again, you have to squash it together too much then, and all you get for it is a watery middle or worse yet, unmelted ice in the middle. :eek:
Also, when you cook a burger, follow what I like to call the "Beatles method" of cooking: let it be. Don't keep squashing it down, don't keep moving it around. Just put it on the grill, give it a 90-degree turn halfway cooking the first side, flip it over GENTLY, and just let it finish cooking without squashing it.
Some people recommend making a dimple-like impression in the middle of each flat side, to facilitate the inside cooking, if you make really thick burgers. I don't think that should be necessary, but then I don't like trying to eat burgers wider than I can open my mouth. :D
Finally: chuck is good. High fat content in the meat is good. Rare is good (just be sure of the cleanliness of where the meat came from; if possible, grind your own). A burger from meat that is too lean and overcooked will never be a juicy burger.
"Notorious stickler" -- The New York Times, January 4, 2004