Tenderness in steaks is a variable thing. Some cuts are always more tender than others, but differences between grades and individual differences between animals can make one steak notably more tender than another, even when they appear exactly alike. Fortunately, there are several ways to tenderize steaks before cooking them.
One of the most widely recommended methods of tenderizing steak is to leave it sitting overnight, or at least for a few hours, in an acidic marinade. The acid helps break down -- or "denature" -- the tough protein strands in the steak, tenderizing it and making it more flavorful. Unfortunately, the tenderizing effect of the marinade is confined to the surface of the steak and does little for the inside. Marinating is still a valuable technique, though. The improvement in flavor is worthwhile, especially if you add seasoning mix to the marinade, and the tenderizing effect can be helpful with thin steaks.
Fruit as Tenderizer
Every piece of meat contains natural enzymes that begin to break it down, over time. That's why aged beef is more tender than fresh beef. Many tropical fruits -- including papaya, pineapple, kiwi fruit and figs -- contain enzymes that work in much the same way. Rather than a conventional marinade including wine or vinegar, it's possible to effectively tenderize meats by layering them with these fruits. Like a conventional marinade, this works best on the meat's surface, so it's best with thin steaks. The other challenge is incorporating or concealing the fruit's flavor. A sweet, tangy barbecue sauce will usually do the trick.
Commercial tenderizers are dry powders that can be sprinkled over a steak before it's cooked, usually 20 to 30 minutes ahead of time. The active ingredient is enzymes extracted from tropical fruits, usually the papaya enzyme called papain. These are very concentrated and if used too heavily, they can give the meat an unpleasantly soft, almost mushy texture. They are, however, more convenient to use than the fruit itself, because the powdered tenderizer does not have a fruit flavor.
Tenderizers and marinades can improve a tough steak, but they work better when they get a little help. Invest in a food syringe at your local kitchenware store and use it to inject meat tenderizer into the steak in several places to tenderize it from the inside out. Another approach is to pound the steak with a meat mallet before marinating or sprinkle it with tenderizer. That exposes more of the tough muscle fibers to the tenderizer. Better yet, many butchers will run your steak through a tenderizing machine at the time of purchase, if you ask them.