Here is what I have learned to make some of the best burgers I have ever had:
-Grind your own.
-Don't overwork the meat. Gently make a pile of ground beef on a flat surface, then form it until it just comes together. The looser the pack of the meat, the jucier the burger.
-Don't mix in anything with the meat. If you want to make a "specialty" burger, do so with the toppings.
-Salt and pepper generously just before hitting the heat. If you salt a while beforehand, the burger will start to cure, and you will get a sausage consistency.
-Cook on a flat top. This will help in 2 ways. One, you get a great crust on the entire flat surfaces of the patty, as opposed to just on the parts contacting a grill grate. Two, it is a lot easier to keep your loosely packed patties from falling apart. Grill marks are overrated, and you can still get that grilled flavor by cooking on a cast iron over charcoal. I have also found that, with these methods, my patties don't shrink up and bulge in the center.
My best burgers have used the Serious Eats "Blue Label" blend. It is some ratio of sirloin, brisket, and oxtail. The oxtail is a bit of a pain to remove from the bones, but adds really nice flavor to the mix.