I always throw mine under the broiler, a few minutes each side depending on the thickness. You want the inside to be pretty pink. It is tender this way and makes great leftovers to throw on top of a salad.
Here is one recipe I use. It was originally for a flank steak, but I use it for london broil also.
Marinade
3 T. dijon mustard
1 T. soy sauce
1 T. worcestershire sauce
1/4 cup brown sugar
2 T. fresh garlic, minced
After broiling the meat let it sit for a few minutes. Then slice along the grain. drizzle juices left over on top of meat.