The best marinades make you hungry for more, relying on a good base of acidity to complement the charred quality of grilled meat. Citrus juices make for a great base, as do vinegars and red or white wine. Add some canola or olive oil, and the sky's the limit from there.
Throw in fresh garlic and herbs with your basic acid and oil mixture, and you've got a simple marinade for any occasion. For an Asian twist, step it up a notch and add some ginger and soy sauce. Or, give your marinade a Mexican flare with a touch of red pepper flakes.