The standard recipe for a hamburger (or turkey burger) is to mix a package of French Onion Soup Mix into the meat before you make the patties.
Or, I mix a little olive oil, worcheshire, and A1 steak sauce into the beef. Then, after grilling them, I add some ham and swiss cheese to the tops of them and let that melt a bit to make "Burger Cordon Bleu."
EDIT: Also, don't use low-fat beef (95% lean or whatever) try to use something like 80% lean. They may not be as healthy, but the fat is what adds flavor and texture. Burgers made from ultra-lean beef tend to come out as dense, tasteless bricks of beef.
