To ensure that your London Broil is tender, you can use a meat tenderizer or mallet before cooking it. Be careful not to use the meat mallet too much as you don't want to flatten the meat out too much. Using a meat tenderizer to poke tiny holes into the meat is a great way to tenderize it as the marinade will really soak in. (Some grocery stores will run the piece of meat through their tenderizer for you for free, all you have to do is ask!) If there is salt in the marinade it will soak into the meat and make it extra tender. The longer you marinate the beef, the more tender it will be...and it adds great flavor too! Once the London Broil has finished cooking, let it rest for 5-10 minutes and then cut it against the grain. Thin slices are recommended.