I'm not sure about london broil but have two different way to cook steak that are absolutely incredible.
One - Look for a meat tenderizer and seasoning named "Allegro"
It should be near the barbecue sauce, steak sauce, tenderizers etc. at the grocery store. It is fucking great stuff. You can also use it on ribs. But, typically I use this whenever cooking steaks on charcoal and throw some hickory chips in with it. Marinate for anywhere from 1-4 hours, preferably about 2 though because the tenderizer will soften the meat up.
Two - use olive oil, soy sauce and Montreal steak seasoning. Use about 3 parts olive oil to 2 parts soy sauce and put a lot of the Montreal steak seasoning in the mixture and mix it up as best you can. Let your steaks marinate in that for about an hour. I personally like to use that whenever I'm cooking inside on the stove and can sear (sp?) the meat in a hot pan.
Sorry, that isn't london broil and may not be what you are looking for. If that's the case, sorry! Didn't mean to ruin the thread. Whenever I thought london broil it made me think of filet mignon, tbones and kc strips cause I'm hungry and like to eat!