Salt and Pepper — and Lots of It
The secret to a burger with a lot of flavor is lots of salt and pepper. When you think you've got enough, it's not, keep pouring. If your burgers are coming out bland or the meat doesn't seem to have much taste other than grill smoke, this is why. It's honestly very hard to over salt and pepper a burger. Put so much pepper on it that people will think you're out of your mind. They'll thank you after dinner.
And Worcestershire Sauce? Bread crumbs? Garlic? Keep 'em for the meat loaf.
They're Not as Big as They Look
First time grillers will form patties that look to be a good size but fail to take into consideration that they will shrink on the grill as the fats melt. For a normal sized hamburger people are used to try aiming somewhere between a quarter and a third pound of meat per burger. They're going to look massive in the kitchen but anything less and the cooked burger is going to look minuscule on top of the bun.
Flip Them Once
Flipping a burger only once ensures proper heat distribution. An amateur griller will be flipping every couple of minutes, but this will result in a loss of juices and could cause irregular heating. (Ignore this if you like really dry, over-cooked burgers.)
The actual grilling of the meat is the shortest, and fastest part of the process. Heating up your grill so it's nice and hot before you stick anything on it is one of the most important rules to remember, and it could take up to 30 minutes. Put the patties on the grill and flip them halfway through. That's it. Depending on how hot your grill is and meat temperature preference, it should take about 5 minutes a side.
And for the love of Pete, don't press down on the patties. Sure, it makes an awesome sizzle sound but you're literally SQUEEZING ALL THE JUICES OUT.
Get a Good Bun
The right bun is as important to a burger's flavor as the crust is to a pizza. Don't buy the $1.09 8-pack, try a kaiser roll or even better ciabatta rolls. Cheaper buns tend to be fluffy and dry, so doin't skimp in the bread department.
Keep 'em Healthy
While not directly related to the burger's taste this will keep your guests from getting food poisoning. When not in use, slide your spatula through the grates of your grill until it will balance by itself. With the spatula only a few inches from the heat, this will aid in killing off anything nasty from the raw meat.