Question: What's the best way to marinate meat?
Answer: I'm a purist: I like my pizza plain, my burgers with American cheese, and my grilled steaks with nothing but salt and pepper. But I understand how some folks (namely my wife) get bored with the simple approach. That's when I break out the marinades.
Marinades are mostly a surface treatment: It can take days for liquids to penetrate deeply into meat to enhance flavor. Some marinade ingredients—acids and enzymes in pineapple or papaya, for instance—will tenderize the meat a little, usually during the first couple of hours of marinating. After that, they merely make it mushy.
All of my marinades have a few common threads: a salty component (soy sauce is great; its salt helps meat retain moisture, and it's packed with glutamates, molecules that trigger our savory taste buds); some sugar to enhance browning; a bit of acid to break down tough outer proteins; aromatics (like garlic, herbs, and spices); and oil to evenly distribute those flavors and aromas.
A good basic marinade ratio is one part acid (vinegar, citrus juice, or yogurt), one part oil (a neutral oil like canola), one to two parts aromatics, and salt (or soy sauce) and sugar to taste. Since it's mainly a surface treatment, I make sure mine tastes good before it ever touches the meat—that way you can brush some more on after the meat is finished cooking for an extra injection of flavor. –Kenji Lopez-Alt