Don't marinate in acid like some people tell you to. Marinades don't go far enough to reach the tough fibers down in the meat, so it's useless. Enzymes will work, but they can sometimes turn your meat mushy. To tenderize a steak, try braising it (that is sort of like making pot roast) or pound it with a meat tenderizer. If that doesn't work, slice across the grain very thinly.