Marinades are very effective at tenderizing cuts like flank and skirt steak, as long as the marinade is very acidic. Vinegar, wine, and citrus juices break down the proteins in the meat and make sure it's soft before it even hits the pan.
It's best to limit marinating to an hour, or else the marinade might work too well and make your meat mushy. It's also not advisable to use acidic marinades with chicken, as acids can actually make chicken tougher. If you'd like to anyways, use plenty of olive oil and don't let sit for too long, 30 minutes is usually enough.
Cooking the meat slowly at a low temperature helps soften the connective tissue that binds the meat. If you want to cook meat that melts in your mouth when it is done, cook the meat at medium to low temperatures. This helps enrich the meat with collagen that makes it tender, flavorful, and silky.
A very easy way to tenderize meat is to tenderize it physically. This involves pounding it with a meat mallet, or if you don't have one, lightly scoring the surface in a crosshatch pattern with a sharp knife. This breaks down the fibers in the meat that make it tough. Make sure not to hit it too hard and have it turn into mush, just enough to break through the fibers on the surface of the meat. This works well for tougher cuts of steak, such as chuck.
The best way to tenderize meat is to use a chemical or fruit-based meat tenderizer. If you don’t want to use a meat tenderizer, you can physically tenderize steak by lightly pounding it with a meat mallet. Be very gentle when pounding the steak, the idea is to break the muscles in the meat. Not mash the meat. The meat mallet works for boneless meat only therefore do not hit meat with bones, such as pork ribs with a mallet.