Having done a lot of cooking, professionally and at home, a good, long marinade helps maximize flavor (lean cuts absorb marinades slowly, so go at least overnight), and the key to "tenderness" is thin slices (1/4" or so), against the grain of the meat. At restaurants, we usually "par-broiled" (quick-seared on a broiler) the entire cut, cooled, and then sliced to order and finished on the broiler.
As an aside, all "value" cuts of beef are horribly over-priced these days; to the point where they're not so much of a value anymore, if you consider the time and effort it takes to make leaner cuts taste good. I like a good London Broil, but if reasonably-priced (relative), tender beef is what you're after, go with top sirloin, which will be priced similarly to most Broil cuts, packs a lot of flavor on its own, and takes much less time and effort to make delicious. My two cents for the age of $8/lb. "value" cuts....
As an aside, all "value" cuts of beef are horribly over-priced these days; to the point where they're not so much of a value anymore, if you consider the time and effort it takes to make leaner cuts taste good. I like a good London Broil, but if reasonably-priced (relative), tender beef is what you're after, go with top sirloin, which will be priced similarly to most Broil cuts, packs a lot of flavor on its own, and takes much less time and effort to make delicious. My two cents for the age of $8/lb. "value" cuts....