Unfortunately, in the real world, marinades have their limitations. Contrary to popular belief, a marinade will not tenderize a cut of meat. But that's all right, it can still impart great flavor. Essentially, a marinade functions as a "dressing" for meat. Like a salad dressing, it's typically composed of an acid (vinegar, citrus juice or wine) and a little oil, with herbs and aromatics added to punch up the flavor.
And like a salad dressing, a marinade should be thought of as a seasoning. Most marinades have neither the acidity needed nor the time required to fully "tenderize" a piece of meat, denaturing the protein bonds. (You probably wouldn't want that anyway, because the meat would then be soft and gummy, rather than truly tender.)
Furthermore, marinades will penetrate only the outer one-eighth to one-fourth inch of the surface of the meat. But that's fine too. Grilling almost always calls for smaller pieces of meat, so you'll still get some of the marinade's flavor with almost every bite.
In fact, if you're making marinade for the first time, consider using salad dressing as an inspiration. For a flavorful Mediterranean marinade, combine equal parts balsamic vinegar with olive oil in a large bowl, then whisk in a little mustard. Taste, then maybe bump up the acidity with some lemon juice and zest, and whisk in a little chopped rosemary and garlic. If you want, add some chopped chives, capers and onion -- they're ingredients you might normally throw in a salad and will work wonders enriching a marinade.
Remember to balance flavors: You can't just throw anything into a marinade, or insert insane amounts of a special ingredient. Restraint is key: "More" almost never equals "better" in the kitchen.
Time it right
How long you marinate will depend on the kind of meat you're using. Keep in mind that because the marinade doesn't penetrate very deep, there's not much advantage to be gained from a long soak.
Fish will marinate quickly. Figure a maximum of an hour or so for fillets or shellfish, even less if the marinade is particularly acidic (you don't want to wind up with ceviche).
Chicken can marinate a bit longer, as the tissue is more dense than fish. Marinate skinless pieces generally for up to four to six hours; skin-on pieces can go for up to six hours or even overnight. (Marinades have trouble penetrating the fat in the skin, thus allowing more time.)
Beef, lamb and pork will typically take the longest: You can go 6 to 8 hours, up to overnight for steaks, chops and kebabs.
When marinating, be sure to use a nonreactive container, because the acids in a marinade will react with some metals, such as aluminum, imparting a metallic flavor to the meat. Use glass, ceramic or stainless steel.
A large, sealable plastic bag works especially well: Combine the meat and marinade in the bag, then squeeze out all the air before sealing; all surfaces of the meat will be in constant contact with the marinade, the bag will take up less space than a bowl or pan in the fridge, and its use simplifies cleanup.
Now mix it up
Once you've mastered a basic marinade, branch out with additional flavorings and aromatics. Draw from regional cuisines for inspiration, or use flavorings or seasoning blends from various ethnic styles.
For a classic Caribbean jerk marinade, combine rum, lime juice and zest, white vinegar and a neutral oil as a base. Layer the flavors by adding garlic, ginger and finely chopped scallions, then whisk in traditional Caribbean seasonings: peppercorns, nutmeg, cinnamon, cloves and a good dash of allspice.
Sweeten the marinade with just a little muscovado sugar -- this rich brown sugar will give the marinade nice molasses undertones, though you can substitute dark brown sugar in a pinch. Add just enough sweetener to impart flavor, as too much in the marinade might cause the chicken to burn as it grills.
Finally, add some heat: Scotch bonnets are the classic pepper in any "jerk" preparation, though habaneros will work fine too. Carefully seed and stem the peppers, then finely chop them and whisk into the marinade, adjusting the heat to your liking. Keep in mind that the heat in the peppers, much like acid, will open the palate to the other flavors in the final dish.
Or go for a Thai-inspired coconut marinade. Start with a can of coconut milk, and whisk in lime juice and zest for acidity. Layer the flavorings with bright notes from lemongrass, basil, garlic and cilantro. Add some ginger, but grate it for this marinade -- grating the root forces the ginger to expel more juice, resulting in heightened flavor. Sweeten the marinade with just a touch of brown sugar, and balance the flavors with sesame oil to lend nuttiness and depth. Finally, chop a couple of Thai chiles to add some heat.
So who cares if marinades don't actually make meat more tender? If they can make it taste this wonderful, isn't that enough?