Grillmaster Fnord here!
Marinades? Stay away from salt, as it tends to toughen the meat. Use an acidic marinade, such as balsamic vinegar, garlic, and a teaspoon of sugar. Otherwise, I've found ordinary salad dressings to work admirably. Use the ones you can see through (Italian, Catalina, Oil & Vinegar), rather than the creamy style (Thousand Island, Ranch, et cetera). I've even used the juice from a jar of sweet pickles!
Cheap beer also works very well for beef or pork. Use white wine for chicken.
Lemon juice for fish.
Marinate for a minimum of one hour in the fridge - over night is best.
Otherwise, low heat and slow cooking is best.
Finally, NEVER USE LEFT-OVER MARINADE FOR A "DIPPING" SAUCE OR RETURN THE COOKED MEAT TO THE MARINADE!! ! Either use it all up at once, or throw out any leftover marinade to eliminate food poisoning.
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