There are three ways to do this. The one you should use depends on the type of meat and what made it tough in the first place.
Consider the Source
In general, exercise toughens muscles. What you know as meat is mostly muscle tissue. So the more exercise a muscle gets, the tougher the meat. That means that in the case of a steer, the big muscles around the legs and shoulders, which are used for locomotion and supporting the animal's weight, yield tougher cuts such as the chuck, the flank, and the round.
Muscles high up along the back and ribs get less exercise, so they produce the more tenderloin cuts. This is where the expression "high on the hog" comes from, but it applies equally to beef. Muscles also toughen with age, so a younger animal yields more tender meat.
Additionally, overcooking meat, even meat that comes from the more tender muscles, can make it tough. That's because heat causes the proteins in the meat to firm up. Overcooking also basically squeezes the moisture out of the meat, making it dry as well as tough.
Distinguish Tough From Chewy
Tough meat can certainly be chewy. But toughness and chewiness are not really synonymous. Chewiness is related to connective tissue and the length of the muscle fibers.
Connective tissue can mean thick pieces of gristle in between muscles, or it can mean the sheets of fibrous collagen that surround muscle fibers. Either way, connective tissue is chewy. And it only gets chewier when it's cooked improperly.
Finally, not all muscles have the same structure. Muscles are composed of fibers, basically long strands of protein grouped together in bundles, that are in turn wrapped in sheaths of collagen. Some bundles have more fibers in them than others, making the grain of the meat coarser, such as with a brisket. Long, thick muscle fibers will be tougher to chew.
Tenderize Meat With Heat
Meat with a lot of connective tissue can be tenderized by cooking it until the collagen melts away, which starts to happen between 160 and 200 F. When the connective tissue melts, it turns into gelatin, which is soft and jiggly rather than tough and chewy.
This doesn't happen right away; it can take hours. Tenderizing in this manner requires patience, but your patience will be rewarded. Liquefied gelatin coats and surrounds the muscle fibers, giving the meat a moist, succulent texture—even though it's been cooked to well-done.
One of the main techniques for accomplishing this is braising. This is a moist-heat cooking technique by which meat is immersed at least partially in some sort of liquid that is then maintained at a gentle simmer for a period of time sufficient to break down the collagen.
Another method is a traditional barbecue: Air around the meat is heated to about 225 F it's cooked for a long time, sometimes eight hours or more.
Either way, melting the collagen in meat tenderizes it and adds to the flavor.
Tenderize Meat With Brawn
You can also break down the collagen in meat with a mallet. This is a useful technique for tenderizing a steak. There are a number of fancy machines and tools to do this, but the most basic way is with a wooden or metal meat mallet.
Meat mallets usually have two surfaces—a flat side and a side with a lot of little points on it.
Pounding a steak with the pointy side of the mallet will cut up the connective tissues as well as the muscle fibers themselves. This allows a steak with a lot of connective tissue to be cooked over high heat without being too tough to eat.
Steaks tenderized like this are sometimes called cube steaks, because the indentations created by the mallet are shaped like cubes.
Pounding also has the advantage of flattening the meat, which allows it to cook more quickly and more evenly. The longer a steak spends over the heat, the drier it gets. And since dry meat is tougher, preserving the juices will produce a more tender steak.
Tenderize Meat With Finesse
Finally, there's slicing—specifically, slicing thinly and against the grain.
Flank steak happens to have very long muscle fibers, and they run the length of the steak.
You could cook a flank steak perfectly medium rare, but if you sliced it along the grain, it would feel like you were chewing a mouthful of rubber bands.
Slicing against the grain shortens those fibers, which means much less work for your jaws and teeth. Fortunately, steaks that most need to be sliced against the grain are the ones with the most pronounced, visibly obvious grain, so you can easily tell which direction to slice. Even if it takes a moment to orient yourself, it's a moment well spent.
What About Marinating?
Cook it, pound it, or slice it—that's it. Marinating was intentionally left out. One of the most common misconceptions is that you can tenderize a steak by marinating it. However, this simply isn't so. Although it's a great technique for adding flavor, marinating does not tenderize meat.