Honestly, don't grill.
Don't run away!
Hamburgers were originally cooked in pans, not on grills. One of the reasons for this is thta with grilling you lose any fat and moisture that comes out of the burger.
So I use Heston Blumenthal's method now. While I tend to ignore the preparation elements (I have neither the time nor the money to hand-grind my burgers with a specific ratio of this cut to that cut to the other cut), the cooking method is flawless:
Make your burgers.
Over very low heat, place a pan with just a little oil (or bacon/duck fat, which is lovely). introduce burgers to pan. As soon as they release easily from the surface, flip. Thirty seconds later, flip. Repeat until cooked through. (fifteen to twentyish minutes).
This will give you the juiciest, most tender burgers you have ever had in your life. I promise. Essentially what you're doing is basting the burgers in their own fat.
Alternatively, if you have an immersion circulator, cook your burgers sous vide and then just mark on the grill when done. I've had burgers done this way and they are excellent.
Burgers will flare on the grill--that's all the fat (and therefore flavour) being squeezed out by the action of cooking and burning away.