How to make a great hamburger is a question that has bedeviled the nation for generations, for as long as Americans have had griddles and broilers, for as long as summertime shorts-wearing cooks have gone into the yard to grill.
But the answer is simple, according to many of those who make and sell the nation’s best hamburgers: Cook on heavy, cast-iron pans and griddles. Cook outside if you like, heating the pan over the fire of a grill, but never on the grill itself. The point is to allow rendering beef fat to gather around the patties as they cook, like a primitive high-heat confit.

More here from the NYTimes. Not sure it's practical to serve a big group with burgers cooked on cast iron pans unless you happen to have a ton of them, but if you're a regular host of big barbeques, it's not that expensive to get a cast iron griddle for your grill.

The other tip in this article that I've always found to be critical is to use meat that is 20 to 25% fat. In the demonization of fat, too many cooks resort to purchasing the leanest burger meat from stores. If you want to eat healthy, eat a carrot stick. If you're going to eat a burger, eat a burger.